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.
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.