Most gardeners are familiar with the idea of hardiness zones — the USDA hardiness zone map, for example, breaks down regions of the U.S. by their expected minimum winter temperature. The idea is that, for example, in Zone 10a (which I’m in), which is listed as 30-35F minimum temperature, the expected low each winter will not be below 30F. Most plants sold are described in terms of their hardiness zone — a plant that is listed as say USDA hardiness Zone 8b should survive in Zones 8b and above. Zone 10a is almost the warmest you can get in the continental U.S. — there are only parts of Southern California and Southern Florida that are in higher zones. And so you would think by virtue of being in 10a that where I live it’d be possible to grow just about anything.
But common sense says that something is missing in this analysis, as does the fact that there are plenty of plants and trees that survive but don’t thrive here. The climate here is the usual Pacific marine climate that predominates along the California coast — a fair amount of fog and wind, moderate rainfall and essentially no frost in winter, and not a lot of warmth. Even in the peak month of summer — which is September here, delayed by the ocean’s thermal inertia — the average high temperatures barely make it into the low 70s F. By the hardiness zone concept, one might expect it to be a great place to grow things, but it’s only okay. I’ve often wondered what concept might explain why gardens in hotter, more inland areas nearby tend to flourish while those here struggle.
I think the concept I’ve been looking for is Growing Degree Days (GDD), which provides the key missing information. GDD is a strange concept, with a stranger unit of measurement. Instead of measuring temperature, or time, it’s in units of temperature multiplied by time. GDD is often based on 50F or 10C, as follows: take the average temperature of the day, A, by averaging the day’s high and low, and subtract from it the baseline B (usually 50F or 10C): X = A – B. X is the number of degree-days you accumulate for that day. Do the same for each day in the year (or growing season, for annuals) and sum it up (ignoring days that have negative values), and you get a measure of how much growing heat was accumulated by plants. 50F is used as a standard baseline, under the assumption that 50F is the minimum temperature for many plants to grow. Below that temperature they are effectively dormant.
You can easily find your local GDD information using this Growing Degree Days calculator. I put in local info and it was a revelation — it explained to me why tomatoes, peppers, squash, and many other annuals struggle to mature here before autumn weather kicks in — they just don’t get the accumulated warm growing time they need. And I saw that nearby inland locations have a 50% higher GDD50 than along the coast.
For fruits that need more heat to mature and sweeten — say Oranges or Pomegranates — you can select 60F or even 70F as a baseline. There again, I found that we just don’t have the heat along the coast here to grow sweet Oranges — something many gardeners in this area could attest to. Not all fruit trees require high GDD to produce mature, good tasting fruit. And indeed some fruits actually need lower values to produce well — some grapes for instance don’t like too much heat while other cultivars require it. Avocados, as it happens, don’t seem to need much heat to produce good fruit, but they can’t stand too much cold in the winter. Some fruits — like bananas — seem to need a combination of heat and no frost, and so they’re among the hardest to grow outside of the tropics.
So this post isn’t to say that the hardiness zone concept isn’t useful — GDD doesn’t tell you what will survive the winter. But both concepts are needed to determine a) what will survive the winter (hardiness) and b) what will grow well (GDD).