The first half of August is our last chance to sow several vegetables and get crops from them before winter. The second half of August is when we sow most of our winter crops. Depending on your climate zone, your dates might need to be earlier or later than ours. We're in cold-hardiness zone 7, and our average first frost date is October 14. In August 2012 I wrote in my blog www.SustainableMarketFarming.com about Last Chance Sowings. Act now or very soon to provide fresh harvests, storage crops and, in the right climates, some crops to overwinter.
Consider planting these three categories of vegetable crops during late summer and fall:
Warm weather crops that will die with frost.
Cool weather crops that grow well in spring and fall, but don’t thrive in your summer.
Cold-hardy crops to grow over the winter and get off to a fast start in early spring.
Planning and timing are critical – you may not get a second chance with that vegetable, if germination fails the first time. The flip-flop challenge with fall crops is sowing in hot weather, followed by keeping the crop happy in cold weather.
Don’t give up too soon! But be realistic about your chance of success. For crops that need to be harvested before killing frosts arrive, the formula for the last sowing date of frost-tender crops is:
Number of days from planting outdoors until harvest (read the catalog or seed packet)
+ Number of days from seeding to transplant if growing your own transplants
+ Number of days you want to harvest from that planting
+ 14 days “Fall Factor” to allow for the slowing down of growth rate as the weather cools
+ 14 days from your average first fall frost date (safety margin in case you get an early frost)
= Days to count back from your average first fall frost date, to find your last sowing date
With rowcover to throw over on chilly nights, you can risk later sowings. For example, yellow squash takes 50 days from sowing to harvest, and our last planting is 8/5, a whole month later than the above calculation suggests. Towards the frost date, you are just keeping the developing fruits growing, so you don’t need to worry that rowcover prevents pollination – you don’t need to get more flowers pollinated. In many parts of the country, a frost or two will be followed by a few more weeks of warm weather, so getting past the first few frosts is worth the effort. (Unless you’ve reached the exhaustion point we call “Praying for a Killing Frost.”) It’s easy to get harvests for a whole extra month from mature plants you have still alive.
This group includes beets, carrots, chard, spinach, lettuce, scallions, peas, potatoes, Asian greens and other leafy brassicas, turnips, rutabagas and radishes. Fall gives you a second chance to enjoy these crops. The flavor of crops produced during warm late summer days and cool nights can be a delicious combination of succulent crunch and sweetness.
The above formula for calculating last sowing dates for frost-tender crops can be modified for hardier vegetables too. Here's an example: Early White Vienna Kohlrabi needs 58 days from sowing to harvest (line 1). You can direct sow, so line 2 = 0. You can harvest it all at once and store it in your cooler, so line 3 is 1 day. Assuming you don’t want to use rowcover for this, line 4 = 14. Line 5 = 14 also. That all adds up to 87 days. Kohlrabi is hardy to maybe 15°F. The temperature at our farm is not likely to drop to 15°F before the end of November, so counting 31 days in October, 30 in September, plus 31 in August – that’s 92 days already, more than enough. We could sow kohlrabi in early August and get a crop at the end of October, or sow in late August and harvest late November.
We sow fall carrots along with some "indicator beets," and run overhead irrigation at night about every other night until they come up. When we see the indicator beets starting to germinate, we flame-weed the carrot beds. Next day there are hazy rows of green - germinated carrots!
We made ourselves a chart of sowing dates for fall harvest crops so that we don’t have to calculate each time. It helps us ensure we don’t sow too late to get a decent harvest. We made this for brassica crops, but you can use the general method to chart later into the fall or winter for other crops.
Sowing dates for crops with various days to maturity
We don't sow spinach till September, so we're not behind on that yet! Because spinach germinates poorly in warm soil, we wait for temperatures to drop. This "summer" has been extremely cool, but we're in no hurry to start spinach earlier than usual, because of all the other tasks. This year it would probably work. I saw fall dead nettle germinating on 8/4. That's a phenology sign that the soil is cool enough for spinach. I've been recording phenology data here since 2003. 8/4 is the earliest date I have for dead nettle, by a margin of 11 days! It has been as late as 9/1 (2004). I also saw chickweed on 8/18. Fall is coming early this year!
Cold-Hardy Crops To Grow Over Winter
I covered this topic in detail in Growing for Market magazine in September 2010, and in my slideshow Cold-hardy winter vegetables. The gist is “Before taking the plunge, know your climate, know your resources, know your market, know your crops, and when you don’t know, experiment on a small scale.” Useful information includes the winter-kill temperature of the crop you want to grow. Choose hardy varieties, and be clear about whether you intend to harvest outdoors all winter (kale, spinach, leeks, parsnips, collards for us), or whether you want to have small crops going into winter so you can rest during the winter and be first out the gate in early spring, with crops waiting for you.