Peas grow best in cool and humid weather, which is why, where I live in Delaware (USDA Hardiness Zone 7), peas are considered early-spring crops only. Summer rapidly becomes too hot, and fall crops are impossible because we'd need to start the plants during unfavorably hot weather, or so the thinking goes.
My experience proves fall crops of peas are not only possible, but that their flavor is often superior. I believe the techniques I've developed will work for gardeners who live in zones 5 through 8. Farther north than zone 5, the growing season isn't long enough for two crops. Farther south (and west) than zone 8, gardeners primarily plant fall crops because spring fades too quickly into summer.
The fall crop in my Delaware garden is lighter than my spring crop, but because the late peas mature in cool temperatures, they generally taste sweeter than spring peas. Production is a gamble. An early hard frost, particularly when the plants are in blossom, can ruin the crop. But, as with any gamble, if you make sure the odds are in your favor, you're more likely to hit the jackpot. With careful variety selection and some simple precautions, I've had good luck.
The key is to plant only varieties with known tolerance of (or resistance to) heat and diseases.
Powdery mildew is the most common disease of peas in late summer. Other diseases that cause problems include: pea enation mosaic virus (primarily in the Pacific Northwest); bean yellow mosaic virus; common wilt; and pea leaf roll virus. To be cautious, call your local extension office or Master Gardener service and ask which pea diseases are common in your area, then select varieties resistant to those diseases.
Timing. The number following the variety name is the nominal --days to maturity--listed for the variety. The goal in fall planting is to time growth so that the first flowering occurs before the first frost in fall. Depending on the variety, that means planting 70 to 90 days before your average earliest hard-frost date. But because young plants grow slowly in late summer heat, I recommend you add 9 to 14 days to the days listed on the packet (and below). That way, the plants won't blossom (you hope!) before a hard frost arrives.
Before planting late peas, add plenty of compost to the soil to help it retain moisture and stay cool during the summer when the peas really have to struggle. I spread about 2 inches of compost over the planting area and cultivate it in. Avoid high-nitrogen organic materials, such as lawn clippings or manures. Because peas are legumes and fix nitrogen in the soil by root nodules, fertilizing isn't necessary. In fact, overfertilizing fall peas increases the plants' susceptibility to frost.
The ideal temperatures for the germination of pea seeds are between 60° to 65° F for soil and about 75° F for air temperature. At those temperatures, the seeds will emerge in a few days. To hasten germination, or if growing conditions are slightly less favorable, soak the seeds overnight in lukewarm water.
If you've never grown peas before and the soil is not well conditioned, consider using a legume inoculant. These inoculants are beneficial bacteria that encourage the formation of the nitrogen-fixing nodules on the roots of legume plants.
After soaking, drain the peas and sprinkle the inoculant powder over them while they're still wet. Then sow them immediately at a depth of 2 inches.
Sowing peas directly in the garden on an overcast day or during a cool period is preferred, particularly when a few days of rain are forecast. I don't recommend starting seeds indoors and transplanting. During a summer of drought and scorching sun, I started 'Super Sugar Snap' peas indoors in peat pots, but they didn't produce well.
It's often a challenge to get the plants through the first month or so, and the heat sometimes wins if the nights are also hot and humid. You must protect the plants from direct, blazing summer sun, preferably with some form of screen. An easy solution is to plant late peas in the shade of tall garden plants like sweet corn or trellised beans or tomatoes that will be harvested in late summer. But even a strategically placed sheet of plywood or corrugated cardboard suffices.
Water and mulch. The weather is extremely hot and humid here in Delaware when I plant. I keep the bed watered until the seeds germinate, and then I water deeply every week until the plants are well established. Never allow the soil to dry out totally, or you'll drastically reduce production.
Because peas' feeder roots are shallow runners, mulch is essential to keep the soil around the roots cool and moist. When the seedlings are 1 to 2 inches high, mulch the plants with a layer of straw, rotted leaves, or compost. As the plants grow, you can add another layer or two of mulch.
Pests. Aphids are the scourge of the fall pea patch and can spread mosaic virus if you haven't selected disease-resistant varieties. Vines that are 4 to 5 feet tall, like 'Alderman' and 'Super Sugar Snap', are especially tempting targets. Sprays of cold water in the early morning will knock aphids off the plants before a heavy infestation can occur.
Although young spring peas can withstand temperatures as low as 17° F, older plants maturing in the fall can tolerate only a light frost. A hard frost (25° F or below) will kill the leaves, the tendrils, and eventually the plants.
If the first killing frost is earlier than usual and you have a timely warning, protect the plants with a floating row cover, cloches, or even an old blanket. If all goes well, peas will mature when the days are chilly, but harvest will be over before bitter cold arrives and the plants succumb to cold.
Experiment with different varieties until you find the ones that do best in your fall garden. You may occasionally lose a crop, but your successes will far outweigh your losses.
Weldon Burge writes and gardens in Newark, Delaware.