Source: Weather Underground, 10-day forecast for Vienna, VA accessed 10/19/2021.
This is the first year that I specifically planted vegetables in late summer, for fall harvest.
I didn’t adequately anticipate how slowly vegetables grow as we move into fall, here in Zone 7. I’m still growing vegetables, but I’m certainly not growing a lot of vegetables.
Given that growth appears to have slowed to a crawl in my garden, I’d like to have some guess as to just how slow a crawl that is.
The core issue is the yield (per unit of time) from the fall garden, compared to the summer garden. Then, once I know that, it comes down to a question of value. Is a fall vegetable garden worth the effort? Would it be worth the effort to rig up some sort of greenhouse-type setup for my raised beds, to keep the temperature up as we move further into fall?
My short answer to the first question is that by October, plants are growing about one-third as fast as they did back in July. So while it is possible to get some yield from an open-air garden in the late fall, it’s not possible to get a lot of yield. Both sunlight and warmth are in short supply.
Sunlight and warmth
We all know that plant growth slows down in the fall, but it’s hard to find a good empirical estimate of just how much it slows. Accordingly, I’m going to work up a quick theoretical estimate.
The theory isn’t that hard to grasp, as long as you can shake loose of any fuzzy-headed notions about vegetables.
Leave behind all that sappy miracle-of-life, beauty-of-nature stuff. Vegetables are biological robots programmed to use energy from sunlight to power a complex series of chemical reactions. The end product of which is — with luck — food. (At least, from my perspective. I’m sure the vegetable itself would have other things in mind, if it had a mind.)
Chemical reaction speed is temperature-dependent. Typically, chemical reactions go twice as fast for every ten degrees Celsius (or 18 Fahrenheit) increase in temperature. Or, in this case, half as fast for every 10 degrees C reduction in temperature.
(Interestingly, the one empirical estimate I found lines up quite nicely with that. In Great Britain, all other things equal, a 1 degree C increase in temperature (in a cross-section of areas, in a northern temperate climate) was associated with a 10 percent increase in plant productivity, per this reference.)
For the Washington DC area, using monthly average temperatures, October averages about 21 degrees F cooler than July. That’s just a bit more than the 18 F typically associated with a doubling of the reaction rate.
The actual impact of temperature on plant growth is obviously far more complex than the simple impact on a single chemical reaction. For one thing, plants use different catalysts (enzymes) depend on temperature, partially mitigating the impact of temperature changes. For another, photosynthesis ceases somewhere in the high 90’sF, and the plant stops growing altogether. For yet another, water vapor pressure affects the plant’s ability to use water from the soil. And so on.
Those caveats aside, as a rough approximation, all other things equal, I’d expect my October garden to grow about half as fast as my July garden. Based solely on the difference in average temperature.
Solar energy is highly dependent on the season. In theory, that’s not just due to the angle of the sun and hours of sunlight, but should also reflect average cloud cover by month. Relying on the National Renewable Energy Labs PVWATTS calculator, October insolation in my area is 58% of July insolation.
That might somewhat overstate the difference, as plants can turn their leaves to be more nearly perpendicular to the direction of sunlight. Thus, as the sun angle declines in fall, plants may (slightly) offset some of the loss by turning leaves to be more nearly vertical. I expect that this effect is modest.
Combining the two, I would then expect my October garden to grow (58% x 50% = 29% as fast as my July garden. That’s from less solar energy, input to a slower set of chemical reactions.
Let me just round that and say three to four times slower. Which implies, I think, about one-third to one-quarter the yield of edible calories per unit of time, for the late fall garden compared to the summer garden.
I haven’t kept detailed notes, but that seems vaguely in line with my observations. In the heat of summer, I had to pick my sweet pepper plants every other day or I’d end up with some excessively large peppers. Now, a couple of times a week is more than adequate. I grew romaine lettuce in the spring, and had harvestable heads within about 45 days. I’m growing the same lettuce this fall, but after about a month in the ground (Post #G21-052), it’s still at the “baby greens” stage, and not nearly worth the effort to pick it.
Conclusion
So, live and learn. It’s obvious that things grow more slowly in the fall. I’m sure experienced gardeners would have known just how slowly the fall garden grows. I had to plant a few things for fall harvest for that really to hit home.
The entire exercise has been helpful. Without putting numbers on it, it’s all too easy just to stick to the plan without paying attention to the outcomes.
In this case, I had planned merely to protect my lettuce from frost. That would require covering and uncovering the beds daily once we start having nighttime frosts.
But now, looking at the first month of growth, I think that’s going to be mostly a waste of effort. Without some sort of greenhouse-type setup to warm up those beds, I’m not going to get much in the way of yield. I’ll end up dragging that frost cover on and off every day so that, in the end, I can grow enough lettuce for a salad or two.
My conclusion is that I should either build a little hoop-house greenhouse on top of that lettuce bed, or just leave it alone and let the frost take it when it comes. Otherwise, with the lack of warmth and lack of sunlight, I’ll spend a lot of time fussing over some stunted lettuce plants that are never going to pay me back for the effort.
I think the moral of the story is that if you’re going to grow vegetables in cold weather in Zone 7, you should either put some serious effort into it, or leave it be. In hindsight, my half-baked approach (freeze protection and nothing more) now looks like it would entail a lot of work for little yield.
