Potato (Solanum spp.) | Andean vegetable | Tuber

(This growing guide is an excerpt from The Cultivariable Growing Guide, available in this store and as a Kindle book on Amazon.)

How to Grow Potatoes from True Potato Seed (TPS)

potato-illustrationAbout Potatoes

Growing

Harvest

Propagation

Diseases

About Potatoes

Introduction

This guide is a little bit different.  I probably don’t need to tell you what a potato looks like or how it is used.  You could fill a library with books written on everything from the history of the potato to laboratory techniques for the manipulation of its genome.  There is no sense in rehashing topics that have been covered many times already, in depth, by people who are far more authoritative than I will ever be.  So, I am not going to describe the characteristics of commercial potatoes or provide instructions for growing from tubers.  Instead, I am going to focus in this guide on growing potatoes from true potato seed (TPS) and evaluating, propagating, and maintaining what you produce.

Although the potato is a common crop, the types that most of us are familiar with are only part of a large range of varieties with different characteristics and uses.  Almost every potato that you have ever seen at the grocery store is of Chilean origin, a group of potatoes that were introduced to Europe in the 1800s.  The ancestors of these potatoes are still grown in the Andes, but remain almost unknown in the rest of the world.  Andean potatoes come in an amazing range of shapes, colors, and flavors, but the only way that most of us will experience them is by growing from seed.  So, growing potatoes from seed not only allows you to breed your own varieties, but also opens up the possibility of exploring these minor relatives of the modern potato.

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True potato seeds

Most people have probably never thought much about how potatoes are grown.  The majority of the people I know can be divided into two camps: those who think that potatoes are planted from seeds and those who believe that there is no such thing as a potato seed.  In fact, potatoes are usually planted from tubers but also can produce seeds.  Potato tubers for planting are often referred to as “seed tubers,” which complicates discussion.  I will not use that term in the rest of this guide; wherever you see the word “seed,” I am referring exclusively to true potato seeds.  In general, you will need to use the acronym “TPS” to find additional information about growing potatoes from seed, since the seed tubers are widely referred to simply as “seed”.

Potatoes belong to the family Solanaceae, the same family as the tomato.  If you have grown both plants, you have probably noticed their similarities.  Unlike tomatoes, which have small flowers and large berries,  potato flowers are large and the berries are small.  Although they are rarely seen in hot or dry climates, potato plants both flower and produce berries that look like small, usually green tomatoes.  Each berry contains up to several hundred seeds, although many varieties produce much smaller amounts.  These berries are sometimes called “seed balls”.

The range of potato plants that even experienced gardeners have seen is pretty small.  There are roughly 50 varieties that are commonly grown in North America and they fall mostly into the category of russet baking potatoes, red and yellow boiling potatoes, and fingerlings.  If you grow from seeds, you will discover that potato plants can grow up to five feet tall, that some of them have partly purple foliage, that they have flowers of many different colors, and that the tubers have a range of skin and flesh colors that extends well beyond the white, yellow, and red potatoes most frequently seen in grocery stores.  Growing potatoes from seed can be a source of both joy and frustration because, in addition to the wide diversity of form and flavor, you will encounter problems that are never seen in commercial potatoes.

potato-comparison

Potatoes of Andean origin often have irregular shapes and surfaces

There are literally thousands of varieties of potatoes.  The majority of them are only grown in their native region in the Andes.  The potatoes used in the rest of the world have emerged from a relatively narrow genetic bottleneck as a consequence of breeding them for performance in other climates and with traits suitable for industrial agriculture.

Potatoes primarily come in two different  genetic arrangements: diploid and tetraploid.  Regardless of whether they are diploid or polyploid, all but a very few experimental varieties are hybrids.  As hybrids, they don’t grow true from seed.  Every potato plant grown from seed is different.  Varieties can only be maintained by replanting the tubers.  Therefore, the primary use of true potato seed is the creation of new varieties

Diploids are almost exclusively grown in the Andes and there are no commercial diploid varieties in North America.  Diploid potatoes are small and usually have lower yields, but they compensate for this with a great diversity of skin and flesh colors.  Many of them have flavor that is superior to commercial potatoes.  In addition to the tubers, the plants are typically smaller and have narrow foliage compared to tetraploids.  The seeds are usually smaller than those of tetraploids as well (Simmonds 1963).

Almost all commercial potatoes outside the Andes are tetraploid.  Tetraploid potatoes are usually larger, higher yielding, and boring.  No, that’s an exaggeration.  Actually, there are a great many exciting tetraploid potatoes… in the Andes.  The potatoes of commerce in the rest of the world are big and uniform but not nearly as fun as the wide diversity of shapes, colors, and flavors that you can grow from seed.

Potatoes have two growth habits: determinate and indeterminate.  Determinate potatoes usually have early and mid-season maturities.  They grow to a certain size, set tubers, and then senesce uniformly.  Indeterminate potatoes will continue to grow for as long as conditions allow, producing very large plants and, in some cases, very large harvests of tubers.  This is not a useful trait for big agriculture, since it makes it difficult to time the harvest.  Where indeterminate varieties and big agriculture intersect, as is the case with the classic variety ‘Russet Burbank’, the solution usually involves killing the crop with herbicide in order to harvest while weather is still favorable.  Indeterminacy is often a very good trait for small growers who are not reliant on mechanical harvesting and no herbicides are needed.

Potato Classification

Potato taxonomists have divided up domesticated potatoes under a number of different schemes over the years.  This is good insofar as it shows the progress of our understanding of the evolutionary relationships between varieties.  It is, however, rather confusing for the amateur potato grower, because all of these systems are still used.

The two main classification systems that are used in potato literature over the past fifty years divide the different potato phenotypes into either nine species and two subspecies or eight cultivar groups underneath the single species, S. tuberosum.  Although these systems are still commonly used, modern genetic analysis has made them both obsolete.  Unfortunately, nobody is going to go back and revise all of the books, research, genebank identifications, and other material that relies upon the previous classification systems. You need to be aware of both the historical and modern systems of classification for now.

Historical Divisions of Cultivated Potatoes

Tuberosum

S. tuberosum subsp. tuberosum or S. tuberosum, Tuberosum group.

Tetraploid.  The modern potato, which probably originated in southern Argentina and Chile from a cross between an Andigena potato and a wild relative (Grun 1990).  This group of potatoes was introduced to Europe about 150 years after Andigena types and was bred intensively following the destruction of many of the existing andigenum cultivars by late blight.

Andigena

S. tuberosum subsp. andigenum or S. tuberosum, Andigenum group.

Tetraploid, originating probably from a natural hybridization between a stenotomum potato and a wild relative (Grun 1990).  Andigena potatoes are considered to be the immediate ancestor of modern potatoes. Size and yield are generally smaller than modern potatoes.  They are considerably more diverse though, with a wide range of colors and shapes.  Taste is often superior to that of modern potatoes. Unfortunately, they are rather vulnerable to disease.  This was the group of potatoes originally introduced to Europe, before late blight developed and made it infeasible to continue growing them.

Stenotomum

S. stenotomum or S. tuberosum, Stenotomum group.

Stenotomum potatoes are mostly diploids.  Unlike the Phureja group, they have dormancy.  They are grown at higher elevations in the Andes and many have some frost resistance (NRC 1989).  Stenotomum tubers tend to be long, often with an irregular surface, and colors are primarily white, red, or purple.  Frost resistance and dormancy make this group of potatoes particularly interesting for breeding for short season climates.  Stenotomum now also includes what was previously known as S. goniocalyx, commonly known as papa amarilla, a group of small potatoes with yellow skin and flesh, known for their particularly rich flavor (NRC 1989).

Phureja

S. phureja subsp. phureja or S. tuberosum, Phureja group.

Phureja potatoes are mostly diploids.  Few have any significant dormancy; very shortly after the tubers mature, they begin to sprout again.  This is a useful trait in the warmer eastern side of the Andes, where they are grown between 7000 and 8500 feet.  Because frosts are rare at these elevations, there can be two or three crops per year.  Lack of dormancy is generally an unsuitable trait for growing in North America, where the tubers can be exhausted in storage before the weather is suitable for planting, although there are techniques that can be used to grow such potatoes successfully, particularly in relatively mild climates like the maritime Pacific Northwest.

potato-berries

Potato berries

Phureja potatoes are usually small, two to three inches (5 to 7.5 cm) in diameter at the most, but they are colorful, often have unusual shapes and deep eyes, and have excellent flavor.  They also often have superior disease resistance.  They can be hybridized (with a low success rate, but one still manageable by amateurs) with tetraploid Solanum tuberosum, usually producing tetraploid progeny (Grun 1990).  The tetraploid progeny sometimes have good dormancy.  They require short days for flowering.  They are grown at lower altitudes in the Andes and do not exhibit frost tolerance.

Phureja potatoes may have been bred from stenotomum potatoes, with the goal of eliminating dormancy (NRC 1989).  Hundreds of varieties exist in the Andes.

Modern Divisions of Cultivated Potatoes

As noted earlier, modern genetic analysis is wiping away all of the historical categories.  I introduced them first because there is such a huge body of literature that uses them that you will probably continue to find them more useful than modern classification for a long time to come.  Modern classification lumps the four categories from the previous section into two cultivar groups under S. tuberosum: Andigenum and Chilotanum.  There are also additional species recognized for the higher polyploids, but I’m sticking to the basics in this guide.  See Spooner (2007) for a more detailed treatment.

Andigenum

This is the group of Andean potatoes and includes everything that was previously included in the Andigena, Stenotomum, and Phureja groups.  This includes diploid, triploid, and tetraploid varieties.  Most are short day varieties.

Chilotanum

This is the group of tetraploid potatoes of Chilean origin.  This includes most modern potatoes, although modern potatoes are now commonly hybridized with more distant relatives.  Most of them are day neutral types.  

There are other groups of cultivated potatoes and roughly 100 edible and non-edible wild relatives, but those are outside the scope of this guide.

Growing Potatoes from Seed

Why Grow Potatoes from Seed?

Growing potatoes from seed adds work and complexity to what is normally one of the simpler plants to grow in the garden.  Toss a few tubers in the ground and a few weeks later, you should have thriving potato plants.  Growing potatoes from seed is similar to growing tomatoes from seed, which is a task that many people outsource by buying starts.  So, why go to the effort?

As a Supplement to Tubers

Have you ever thought that you would like a potato a little bit different than those available for planting?  Maybe you need resistance to a particular pest or disease.  Maybe you like the flavor of a variety but wish that it produced a bit better.  Maybe you would like to experiment with colors and sizes that aren’t commonly available.  If so, you could consider breeding your own potatoes from seed.  Once you find a potato that you like, you can just keep replanting the tubers every year.  Over time, you can build a collection of your own varieties, unique to your garden.

Instead of Tubers

Do you have problems with potato diseases, but hate buying certified tubers?  Do you like to experiment more than you value a predictable harvest?  Would you rather have a bowl full of tubers of all shapes, sizes, and colors, rather than uniform potatoes that look just like what you can buy at the store?  If so, you might forget about planting from tubers entirely and just grow from seed every year.

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Blue foliage may indicate blue flesh

Although I now keep a lot of tubers for continued breeding, for several years, I grew potatoes only from seed.  It was a surprisingly successful experiment!  We never lacked for potatoes to eat.  Many of the plants were low yielding, but many were also high yielding, so it balanced out.  Production was about the same as I achieved growing commercial potatoes from tubers.  I rarely found inedible, bitter potatoes, although there were a few that tasted pretty bad.  It should be noted that I live in a very potato friendly climate, so results may not be so good elsewhere, but I definitely think that this is an experiment worth trying.

Climate Tolerance

Potato plants flower and set seed most readily in cool, humid conditions.  This is a commonality of Andean crops, which originated in cool, tropical highlands.  Some varieties will set seed even in hot, dry conditions, but the widest range of varieties will flower and fruit in maritime climates.

In many climates, it will be much easier to start with seeds produced by someone growing in more favorable conditions.  Once you have seedlings with a wide range of characteristics, you are likely to find a few that set seed in your climate more readily than the commercial varieties.  You can then save seeds from those plants and you will be on your way to a better adapted potato for your location.

Photoperiod

One of the great breakthroughs that allowed the potato to become one of the world’s staple crops was the development of varieties that are day length neutral.  In the Andes, many potatoes are short day plants; they don’t begin to form tubers when there are more than roughly twelve hours of daylight.  This is not an impediment near the equator, where day length changes very little over the course of the year, but at higher latitudes, short day varieties don’t begin to form tubers until the end of September, by which time frosts are approaching in many areas.

When you grow commercial varieties of potato, you are guaranteed that they will be day neutral, but when you grow potatoes from seed, particularly if they are diploid, there is a good chance that they will turn out to be short day plants.  In colder climates with a short growing season, you will probably want to discard all of the short day plants, since the odds of producing tubers are low.  However, in climates with a long growing season, short day plants can actually turn out to be good performers.  They often produce very large vines and, although they form tubers late in the growing season, they can leverage that huge plant to form heavy yields quickly.

Flowering is also influenced by day length.  In general, long days are most suitable for potato flowering.  Seed also has stronger dormancy when produced under long day conditions (Taylorson 1982).  For this reason, seed produced at higher latitudes is best stored for a year or more before sowing, although there are ways to improve germination in dormant seed, discussed below.

Propagule Care

Potato seeds are typically long lived, retaining good germination for at least fifteen years at 70° F (20 C), showing almost no reduction in germinability (Barker 1980).  By reducing storage temperature to 40° F (5C) or less, they may retain acceptable germination for 50 years or more.  Potato seeds can even survive in the field to germinate as volunteers for seven years or more (Lawson 1983), something worth considering if you will be producing a lot of seed.

A Tuber and Seed Problem

You have two possible routes to starting your adventure in growing potatoes from seed: you can grow plants from tubers and use them to produce seed or you can obtain seeds to start with.  If you do not have an existing collection of tubers to work with, I recommend starting with seed, as it will save you time, have a far lower risk of introducing potato diseases, and should give you much greater genetic diversity to work with.  You should also consider that many modern varieties are male sterile.  They will set seed if pollinated by another variety, but they will also transmit the male sterility to the next generation.  This is another good reason to start from seeds that have been produced by a population that does not include male sterile varieties.

Planting

Potato seed is a fickle creature.  Some germinate easily, in a short amount of time and with good uniformity.  With others, you will be certain that the seeds were duds, only to have a seedling pop up a month later, and then another in a couple of weeks, followed by three or four in a flush, and then a long waiting period again.  In some cases, the reason is genetic. In others, it is environmental.  Potato seeds are covered in germination inhibitors and if the seeds are insufficiently cleaned, they can remain to interfere with your seed starting.  Also, these substances break down over time, so fresher seed often has more irregular germination than old seed.  

Potato seed that is less than one year old is considered to be dormant and germination is more difficult than in older, non-dormant seed.  Some varieties like conditions a little warmer and some a little colder.  Some are just less domesticated and are holding onto the risk hedging strategy that is present in most seeds of wild plants; spreading germination over a long period ensures that at least some will grow under favorable conditions.

potato-flowers

Potato flowers

Sowing

Start potato seed indoors about eight weeks before you will be ready to plant them.  This can be eight weeks before your last frost, or any time thereafter.  In that time, you should be able to produce seedlings that are about five inches (12.5 cm) tall and ready for transplant to the field.  Once the seedlings are eight weeks old, you need to move fairly quickly to get them transplanted to the field as any with early maturities will begin to form tubers and further growth is limited once tuberization begins.  Sowing as soon as frosts end may be important in hot summer climates, where you want the plants to be able to grow in cooler spring weather.  In a cool summer climate, you can start potatoes from seed as late as August and still produce a crop.

The optimum constant temperature for potato seed germination is about 60° F (16 C) (Lam 1968, Gallagher 1984, others).  Alternation between 12 hours of 65° F (18 C) and 12 hours of 55° F (13 C) is more effective at germinating dormant seed (Lam 1968).  At 60° F, most potato seed varieties begin to germinate within ten days.  They may germinate uniformly, with most of the seedlings emerging by the twentieth day, or you might get one seedling each week for months. I recommend starting seeds over a heating pad in order to achieve uniform temperature.  In general, the more uniform the temperature, the more uniform the germination.  When the temperature drops significantly at night, germination can be slowed substantially.

For improved germination, pre-soak seed in room temperature water for 24 hours.  Germination can also be greatly increased by exposing the seeds to giberellic acid (GA3). A 24 hour soaking in 50 ppm GA3 can produce more than 90% germination at 60° F (Lam 1968).  A 24 hour soaking in 2000 ppm GA3 will break the strongest dormancy (Simmonds 1963).  I wouldn’t hesitate to use GA3 to germinate irreplaceable seed, but I think it is worth considering whether regular use of GA3 might introduce dependence.  When breeding plants, I try to avoid using techniques that I don’t intend to employ permanently.

Press seeds into the surface of your medium (any soil or soilless medium is fine) and then cover just barely with a little bit of fine material.  Potato seed germination is inhibited by light, so it is important to cover them, but the seedlings are fairly weak and cannot push their way through a thick layer of soil.  Put the seeds under lights two to four inches (5 to 10 cm) above the surface of the soil.  The daily lighting period should be at least 14 hours; I prefer to use 16 hours on and 8 hours off, in most cases.  This will ensure that they get light as soon as they break the surface.

Keep the surface of the soil moist until you see seedlings beginning to emerge.  At that point, you have two choices: you can prick out the seedlings and transfer them to another container or you can continue to grow them in the original containers.  If you don’t transfer them, withhold water, allowing the surface of the soil to dry out before watering again.  I prefer the first method because the conditions necessary for good germination and the conditions necessary for maintaining seedling health aren’t exactly the same.  Seedlings will usually perform best at temperatures between 60 and 70° F (16 to 20 C), but will tolerate a wider temperature range as they become established.

Transplanting

You can transplant once or twice.  There are advantages and disadvantages to both methods. Whichever way you choose, allow the soil to dry out in between waterings enough that it begins to shrink away from the walls of the container, but not so much that the seedlings wilt.  Water from the bottom.  You should water just enough that all of it is taken up into the containers.  Don’t let the plants sit in a puddle.  Managing wetness is crucial in avoiding damping off.  A little water stress causes potatoes to form more extensive root systems, which is a big benefit when you transplant to the field (Wagner 2011).

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Cleaning potato seeds

Initial Transplant

Doing an initial transplant is optional, but can produce superior results and usually makes your life easier when transplanting to the field.  Prick out the seedlings and plant into small cells in flats.  This forces the seedlings to produce compact root balls (Wagner 2011).  Although you can also transplant them into undivided flats, or containers of any sort, cells avoid the problems of spreading, tangled roots that result if you sow in open flats.

You can also transplant into a seedling bed if you have a greenhouse or live in a suitably mild climate.  This is my preference.  I plant seedlings in a bed with a six inch spacing in all directions.  The root ball is not nearly as compact as those of plants grown in containers, but because the plants are not entangled, you can easily pull out the whole, fist sized root balls for transplanting to the field.

Field Transplant

Once your seedlings are about four inches (10 cm) tall, you should begin hardening them off.  You can find an unlimited number of methods for hardening off, but the main idea is that you will gradually introduce the seedlings to outdoor conditions.  I like the doubling method: one hour outside the first day, two hours the next, then four, eight, and sixteen hours.  After that, the plants are ready for transplant.

If you chose not to do an initial transplant, then your seedlings will be crowded together and preparing them for field transplant will take some work.  This involves a lot of disentangling and usually some losses.  The easiest way is to soak each container in water for about an hour and then use a hose to spray the wet soil off of the roots.  You can then pull apart the seedlings and plant them in the field.

You should transplant into trenches.  If you did an initial transplant, the trench needs to be only about two inches (5 cm) deep, while seedlings that are undergoing their first transplant need trenches about four inches (10 cm) deep.  Plant the seedlings so that all but the top leaves are buried in the bottom of the trench.  As the seedling grows, fill the trench, always keeping the top set of leaves exposed.  Once the trench is full, the plant will have enough space to set a full yield of tubers.  Alternatively, you can hill on top of flat ground, but make sure that your hilling spreads in at least a 12 inch (30 cm) radius around the plant.  18 inches (45 cm) is even better, since seed grown potatoes can have very long stolons.

Harvest

As with potatoes grown from tubers, it is time to harvest after the tops die down.  Once the tops die, the skins of the tubers toughen, which reduces the amount of damage that you do when digging them up.  Think about how you want to evaluate them before you start harvesting.  You might want to take note of things like the length of the stolons that can’t be evaluated later.  Taking pictures of each plant as you harvest can be a useful record.  If you want to measure yield, then you will need containers for each plant so that you can weigh them individually later.

Selection

At some point, you will need to evaluate the potatoes that you have harvested and decide which, if any, you will grow again.  Some criteria are obvious: color, shape, and flavor are all fairly easy decisions.  You might also want to think about maturity, dormancy, and disease resistance.

potato-new-berry

Berries are revealed when the petals drop from the flowers

Maturity

Early varieties are important for both regions that have short growing seasons and regions where blight arrives in late summer.  You can easily select early varieties in the seedling stage by allowing them to form tubers in the trays.  The early varieties will senesce without ever being transplanted to the field and you can retain the microtubers for planting (Wagner 2011).  Otherwise, they are equally easy to detect in the field as long as you start growing them early enough in the year.  The first to senesce are your early varieties.  Just make sure that they died back naturally and not as the result of disease!

Dormancy

You will want to track how many days your tubers last in storage before they begin to sprout.  The dormancy period needs to be long enough that the potatoes survive until planting time.

It appears that tuber dormancy and seed dormancy are linked (Simmonds 1964).  Varieties that have little tuber dormancy will often also have little seed dormancy and vice versa.  This can be useful for high latitude potato breeders: seed lines can be sown soon after collection to evaluate for dormancy.  Those that germinate quickly can be discarded under the assumption that their tubers will have poor dormancy.

Disease Resistance

The simplest approach to breeding for disease resistance is to eliminate plants that show any symptoms during the growing season.  This can be very effective, but it isn’t necessarily the best place to start.  A wider base of resistance genes can be formed by selecting plants in your first few breeding seasons that are vulnerable to disease, but resilient enough to survive and produce a yield.  Although counterintuitive, this has the potential of producing more resistant plants in the long run (Robinson 2007).  That is an extreme simplification.  For more information, I recommend that you take a look at both the Amateur Potato Breeder’s Manual and Return to Resistance by Raoul Robinson.

Safety

potato-blue-rose-early

Early varieties may show signs of tuber formation by the time you transplant

At some point after harvest, you will be ready to give your new varieties a taste test.  Some people have heard horror stories about potatoes developing toxic levels of the glycoalkaloids solanine and chaconine.  Potatoes that are exposed to sunlight have increased levels of these compounds, which is why you don’t eat potatoes that have turned green.  Some of the wild ancestors of the potato have much higher concentrations of these compounds even without exposure to sunlight and this trait can show up in potatoes grown from seed.

In one famous case, Lenape, a potato bred by the Wise Potato Chip Company turned out to have a level of solanine high enough to occasionally cause nausea and vomiting (Koerth-Baker 2013).  Most people had no problems with it and the flavor was said to be really good.  Interestingly, the bitter characteristics of the glycoalkaloids are a significant component of potato flavor, but if you go too far, the potatoes become unpleasantly bitter and unsafe to eat.

Safety is always a relative consideration.  Some people eat potato tops, which are much higher in glycoalkaloids than the tubers.  They appear to do so without ill effects, so there is probably considerably more tolerance for these compounds in the human diet than the estimated lethal dosage indicates (Phillips 1996).  That isn’t a suggestion that you should try to find the upper limit, but will help to put the danger in context.

No potato that you grow from seed is likely to do you serious or permanent harm when consumed in small amounts.  Solanine is quite bitter, so as it rises in concentration, the odds that you will finish chewing a mouthful decline dramatically.  Potatoes that taste just a little bitter may cause no reaction at all in some people, but unpleasant results in others.  The typical response to a small overdose of solanine is diarrhea.  A moderate overdose takes it up to nausea and vomiting.  A serious overdose can result in confusion, weakness, and unconsciousness (Ruprich 2009).  Symptoms may appear about four hours after consumption (Mensinga 2005).

The best practice for evaluating new varieties would seem to be starting with small amounts.  I cook new varieties in the microwave and eat them plain, so that bitter flavors are not masked.  I have eaten hundreds of potatoes grown from seed, found only a few that were terribly bitter, and I have never gotten sick taste testing them.  Potatoes that aren’t noticeably bitter are probably very safe, although some people are insensitive to bitter flavors.  If you are one of them, you might need to get some help with your taste testing.

Propagation

Pollination

potato-seedlings

Potato seedlings at about one week

If you want to keep growing potatoes from seed, you will need to produce more seed.  There are many ways to go about this.  You can plant a group of varieties that you like and allow them to open pollinate.  You can perform hand pollinations between varieties. With tetraploid varieties, you can also isolate varieties that you like and let them self pollinate.

The first thing that you will need is some potato flowers.  In favorable weather conditions, flowers appear two to three months after tubers or seedlings are planted out.  The most favorable conditions for flowering and berry production are daytime temperatures in the 60s F and nighttime temperatures in the 50s F, accompanied by high humidity.  Some varieties will flower in warmer, drier conditions, but very few will flower when daytime temperatures rise above 85° F and berries tend to drop early in warm weather.  Flowering lasts about two weeks for most varieties.

Diploid potatoes are self incompatible; they require cross pollination with another variety in order to form seed.  In most cases, seed set is much better with hand pollination than relying on insect pollination.

Tetraploid potatoes are usually self compatible and will set seed that has been pollinated by their own flowers, if they have fertile pollen.  That is a big “if”; most tetraploid potatoes of modern origin have pollen sterility problems.  If they have sterile pollen, they require cross pollination to produce seed.  But, there is another problem: the seeds collected from varieties that have sterile pollen will usually also have sterile pollen.  Even when self pollination works and produces progeny with fertile pollen, the practice eventually results in inbreeding depression, which means that you are more likely to get good results by cross pollinating tetraploid potatoes than by allowing them to self pollinate.  This is not always true, but the details are beyond the scope of this guide.  Roughly 80% of the seeds produced by self compatible tetraploids with fertile pollen, even in mixed plantings, are self pollinated.  In order to maximize hybrid seed production, you must emasculate flowers and hand pollinate.

When hand pollinating, you have the choice of performing open pollination or controlled pollination.  Open pollination is the easier choice.  You simply collect pollen from one set of flowers and transfer it to another.  One easy way to go about this is to use an electric toothbrush to vibrate the flowers, with a small container below to catch the dislodged pollen.  You can then use a small brush to paint the pollen onto the flowers that you want to pollinate, or if you have collected a lot of pollen, simply dip the flowers into the container.

Controlled pollination requires emasculating the flower: removing the stamens before their pollen matures.  This procedure is beyond the scope of this guide, but you can find instructions and even videos on the Internet.

When hand pollinating potato flowers, repeating the pollination at least once produces significantly better seed set than a single pollination (Pallais 1985.)  You can store the pollen that you have collected for later use, which may be important if the varieties that you wish to cross flower at different times.  Dry the pollen for 24 hours in a container with silica gel desiccant and it can then be stored at refrigerator temperatures for up to two months without significant reduction in viability (Pallais 1985).

For natural pollination, bumble bees appear to be the most active pollinators in the Pacific Northwest, followed by hoverflies and other small, native bees.

Collection

It takes a minimum of six weeks for berries to mature (Simmonds 1963).  The optimum time for berries to mature varies from one variety to another and I would generally prefer to not harvest berries that are less than eight weeks old.  In warmer climates, berries can mature more quickly.  The berries closest to the main stem produce the largest yields of seed (Almekinders 1995).  These are typically also the earliest berries.

Extraction

potato-final-transplant

Seedling ready for final transplant

Once you have harvested mature berries, you need to extract the seeds.  There are a number of ways to go about this.  For small quantities, you can merely cut berries in half and squeeze out the seeds.  This has the advantage of causing little damage, but would become quite tedious if you are planning on extracting hundreds or thousands of berries.

For large extractions, a blender is often used to break up the berries for about 30 seconds.  This works very well, but it does cause damage to some of the seeds.  The number of seeds that you will get from a blender pitcher full of berries is so large that it probably doesn’t matter, but it has been reported by the International Potato Center that a meat grinder provides a superior result, breaking up berries with very little damage to the seeds.  Another study found that using a mixer with rubber beaters produced the highest percentage of germinable seed (Gallagher 1984).

Once the basic extraction is done, you have two choices for how to clean the seed: fermentation or detergent.

Fermentation for 4 days at 86° F (30° C) produces clean seed (Pallais 1985).  The only problem with fermentation is that you often don’t completely get rid of the mucilage and the seeds have a tendency to clump when drying.  You can speed things up by adding some dry yeast.

Detergent generally produces seed with less mucilage, which clumps less during drying as a result.  This may be an additional layer of protection against surface contamination.  Potato breeder Tom Wagner recommends the following procedure: Rinse seeds in warm (90° F) water until it runs clean, without any slimy strands.  Then soak for 20 minutes in a 10% solution of trisodium phosphate (TSP).  TSP can be purchased at hardware stores.  It is a detergent that is used for cleaning surfaces before painting, among other uses.  A 10% solution can be made by adding 24 grams of TSP to a cup of water.  At the end of 20 minutes, rinse the seed clean.

Once your seed is clean, there are other treatments to consider.  To eliminate surface contaminants, such as bacteria and fungi, you can bleach the seed.  Soak for 10 minutes in a solution of 1 tablespoon (15 ml) of household bleach per 1 cup (237 ml) of water.  You can also possibly reduce the odds of transmitting viruses by heating the seeds.  I don’t usually do this, as I’m not sure about the long term effect on germinability or the efficacy for that matter.  Soak the seeds in 50 C (122° F) water for 25 minutes.  You will need a hot plate or other appliance that can hold a constant temperature.  If the temperature goes higher than 50 C for very long, it will kill the seed.  The objective here is to heat the seeds enough to kill the viruses contained within them, without harming the seed.

Diseases

Although most potato growers are keenly aware that tubers can transmit bacterial, fungal, and viral diseases, most are probably not aware that disease can also be transmitted through seed.  North America and Europe are pretty safe because the worst seed borne diseases are not present (yet), but this is a good reason to exercise a great deal of caution when importing seed from other regions.

Spindle tuber viroid is transmitted in true potato seed (Hunter 1969, Singh 1970) and is probably the most damaging virus that infects TPS.  In one study, 100% of seeds produced by infected plants were infected (Grasmick 1986), so you can image how damaging this viroid could be if it were to spread.

In addition, the oca strain of arracacha virus B, the potato calico strain of tobacco ringspot virus, and potato virus T can be transmitted through seed (Jones 1982).  Cultivariable customers particularly should note that some of these viruses are cross infectious with oca and arracacha.  Great care should be exercised whenever introducing varieties of unknown provenance.

Rhizoctonia and other fungal diseases can be carried on the exterior of the seed (Pallais 1985), a good reason to surface sterilize the seeds with bleach.

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