Just like the merchants who start displaying the back to school fashion lineup when it's still beachwear weather, school gardeners who want enjoy a fall vegetable harvest need to think ahead during summer's heat to cooler times to come. A little planning and planting beginning in midsummer can pay big dividends when students return at the end of the summer by allowing the school garden harvest to continue into the fall or even winter months, depending on your climate and the protection you give your crops.
There are lots of veggies that can be sown from mid to late summer on into early fall to provide tasty eating in the months to come. Beets, cilantro, kohlrabi, broccoli, spinach, carrots, chard, Chinese cabbage, bok choy, radishes, turnips, lettuce, and hardy greens like arugula and mizuna are all cold-tolerant plants that will take light frost. And some crops, such as kale and Brussels sprouts actually taste sweeter when harvested late in the season after they have been touched by frost. In areas with mild winters, like southern California, the Gulf Coast, and warm parts of the Southwest, you may even be able to grow cold tolerant crops throughout the winter.
To figure out when to plant in order to reap a harvest before temperatures drop too low, you will need to do a little simple math. (Sorry – I know it’s still summer vacation!) Start by finding the days to maturity (DTM) for the particular crop and variety you plan to grow. This information is listed on the seed packet and tells you how long on average it takes to go from seed sowing to harvest. For example, Black-seeded Simpson lettuce takes 45 days to reach maturity (only 28 days if you plan to harvest it at the “baby” stage).
Next, figure out the average date of the first fall frost in your area. While tender crops like beans and basil will be killed by light frost (32 degrees F), cool season crops like those listed above will survive until hard frost, when temperatures dip to 28 degrees F or lower. Especially hardy kale, cabbage, collards, and Brussels sprouts can withstand hard frosts, but will usually be killed when temperatures get down to 20 degrees F or lower. To determine when these temperatures arrive in your area, check with your local Extension Service or check out information available online from NOAA.
Okay, now it’s time to sharpen your pencil and get figuring! Starting with the DTM, add on 14 days as a “fall factor.” This takes into account the slower growth of plants as the days get cooler and shorter in fall. Then add in a harvest period, generally between 7-14 days. This will give you the total number of days needed for growing and harvesting your crop. Count back this number of days from the date of your first killing frost and – voila! – you’ve arrived at your planting date.
Here’s an example. Where I live in Vermont, there’s a 50% chance that temperatures will drop to 28 degrees F by October 7. So if I want to plant some of that Black-seeded Simpson lettuce for fall harvest, I add up 45 days (the DTM) + 14 days (the fall factor) + 10 days (harvest period) to arrive at 69 days. Then I count back 69 days from October 7 to arrive at a planting date of July 30 for full size plants and as late as September 8 for “baby” plants. And since the actual frost date varies each year, I’ll plant some seeds even later and keep my fingers crossed – if I’m lucky and frost holds off I’ll get an extended harvest. If you’re fortunate enough to have a cold frame or tunnel to protect your plants, you can push your planting dates forward, often by two to three weeks or more.
So remember, even though the weather is in the 90s and the garden is overflowing with zucchini and tomatoes, when you see the bathing suits go on sale, it's time to think about planting for fall! For more details, check out Plan for a Back-to-School Harvest.