USDA Zones for Andean Root and Tuber Crops

One of the most common types of questions that I get is also the most vexing: What is the USDA zone for <plant>?

USDA Zones have a very specific and narrow application, but they get used for all sorts of inappropriate things.  The only thing that a USDA zone can really tell you is whether or not a given biennial or perennial plant can survive the extreme minimum temperature that occurs in the zone.  For example, I am in zone 8b, which corresponds to an extreme minimum temperature of 15 degrees F.  That sounds about right.  In the 14 years that we have lived here, I have never seen a low of 15 degrees, but I have rarely seen temperatures in the upper teens.  I wouldn’t want to make an investment in perennial plants that take a long time to come into bearing if they can’t survive a low of 15 degrees.  USDA zones are meant to make me think twice about planting out an orchard full of trees that die at 20 degrees F.

Beyond that, USDA zones aren’t very useful.  They have nothing to say about the maximum temperatures that a plant can withstand, or the minimum temperature necessary for good development, nor the water requirements of the plant.  Often, people use zones as a sort of shorthand.  If someone in Zone 8b in Austin, TX can grow a plant, there is a pretty good chance that someone in Zone 8b Tallahassee, FL can also grow it.  These cities are at about the same latitude and elevation and so they have similar conditions.  However, I am also in Zone 8b, all the way up on the coast of Washington, 17 degrees of latitude north of those cities and plants that grow in my climate often do very poorly in Texas.  If you want to know whether or not a plant will perform well in your climate, you need to look at much more than the USDA zone.

So, the title of this post is a bit misleading.  I will give you the USDA zones that I have calculated for the Andean root crops, but they are pretty much useless.  It is unlikely that most people would leave them planted over the winter anyway.  If you are going to harvest a crop before winter, USDA zone doesn’t apply at all.  Figuring out the minimum temperature of a root crop is also tricky business.  Soil type, soil moisture, planting depth, stolon length, mulching, and other factors all affect the outcome.  Shallowly planted potatoes will often not survive a zone 8b winter, but if you bury them in a compost pile, mulch them, or plant them deeply, they can sometimes survive winters all the way down to zone 5.

Rather than look at the USDA zone, I recommend that you look at all of the data in the following chart.  This will tell you not only whether or not the crop can survive your worst winters, but whether they will survive your summers and produce before being killed by frost.

Crop Minimum Time to Harvest

Photoperiod

Short Day

50% Yield

Short Day

80% Yield

Optimum Temp. Max. Sustained Temp.

Frost

Tolerance

USDA

Zone 

Achira

4 months from rhizome

7 months from seed

Day Neutral n/a n/a 75-80F 90F None 9a
Ahipa 5-6 months Day Neutral n/a n/a 75F 85F None 10a
Arracacha 11-13 months from transplant Day Neutral* n/a n/a 75F 85F None 9b
Maca

6 months

Day Neutral n/a n/a 60F 85F Slight 8b
Mashua

3 months

Short Day** 4 weeks 12 weeks 65F 80F None 8b
Mauka

6 months from caudex

10 months from seed

Day Neutral*** n/a n/a 75F 90F None 9a
Oca 5-7 months Short Day 5 weeks 10 weeks 65F 85F None 8b
Potato, Modern 3-8 months Day Neutral n/a n/a 60-70F 85F Usually none 8a
Potato, Andean 3-8 months Short Day 5 weeks 10 weeks 55-65F Varies**** Varies***** 8a
Ulluco 7 months Short Day 6 weeks 11 weeks 65F 80F No 8a
Yacon 6 months Day Neutral* n/a n/a 75F 90F No 9a
* This is not completely settled.
** Except for the variety Ken Aslet, which is day neutral(ish).
*** But not for flowering, which is a short day response.
**** About the same as modern potatoes.  Low dormancy diploids seem to tolerate more heat, as do some Andean tetraploids.
***** Most are about the same as modern potatoes.  Some are slightly frost tolerant and Solanum ajanhuiri, S. curtilobum, and S. juzepczukii are moderately frost tolerant.

The minimum time to harvest is the time it takes the plant to reach maturity under favorable conditions.  This ignores photoperiod.  So, for example, oca can produce a crop in five months, but if you plant in March, you will not be able to harvest in August in North America, because oca also requires short day conditions to form tubers.

Photoperiod shows the day length required to form the roots/tubers.  Some species may also have photoperiod requirements for flowering.  Day neutral plants will form roots at any time of year, but short day plants will not begin to form them until after the Autumn Equinox (~Sep 23).

The short day yield columns show the number of weeks required after the Autumn Equinox to reach the stated yield.  So, for oca, 50% yield is reached five weeks after Sep. 23, about the end of October.  80% yield is reached in 10 weeks, around the first week of December.  I don’t list a value for 100% yield because it is not easy to achieve in my climate and takes a long time.  I see max yield for Andean Crops in late January in favorable years.  Most climates in North America will not reach 100% unless the plants are grown in a greenhouse.  These are my local observations; your mileage may vary.

The optimum temperature is the daytime temperature under which the crop grows steadily and flowers abundantly.

The maximum sustained temperature is the maximum temperature at which the crop can grow indefinitely without heavy losses.  Beyond that temperature, plants grow poorly, become more vulnerable to pests and disease, and often die.  If nighttime temperatures are significantly lower, the plants may be able to sustain daytime temperatures a little higher.

Frost tolerance should be pretty much self-explanatory.

6 thoughts on “USDA Zones for Andean Root and Tuber Crops

  1. Nikki says:

    It is so easy to check your zone and not enough people do it before planting their trees. They wonder why their tree did not make it over winter CHECK YOUR ZONES PEOPLE!

  2. Eli Brandt says:

    This is awesome! I wish other plant suppliers could describe the parameters affecting a plant’s growth like this, instead of just minimum temperature and sun influx. (Or are these presented if you’re a serious wholesale buyer?)

  3. strike2020 says:

    Missing from the USDA Zone System: Growing Season Length; Distribution of Heat During Months of Growing Season/Average Daily Temperatures By Week or Month Throughout Growing Season; Seasonal Growing-Degree-Days; Maximum Expected Growing Season Temperatures; Minimum Expected Growing Season Temperatures; Growing Season Photoperiod By Week (particularly important at my near-60-degree-North latitude)…

  4. strike2020 says:

    To the poster of this excellent blog: A way for readers to effectively print posts for personal reference would be very nice.

    • Davilyn Eversz says:

      Take a screen shot if you have a Mac. Screen Capture App for other OS. You can copy and paste with ease though. Any of these ways will print out.

  5. chris says:

    I am in northwestern California just south of the Oregon border and 50 miles due west of the pacific ocean in zone 8b, elevation 1300′ My winters are very mild, often 25 to 45 degrees in a day, then now March 20, 40 to 70 when sun out, then in summer a 40 degree difference in temperature during a 24 hour period and some plants, like tomatoes, abhor cool nights. so i have decided to grow as many edible plants in pots as i can so i can more them inside/outside to extend my short season on either end (can get a surprising frost at either end). Sometimes temperature goes up to 115, very often in the high 90’s June July & Aug. If you know of any edible plants that might do well here please let me know. And thank you sooooooooooooooooooooo much for your wonderful work!
    chris

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