On average, August has 14-hours of daylight. June has 15, September has 12 and October has only 11 or 10 (www.timeanddate.com). The reduction of sunlight hours helps lower the temperature of the soil so seeds can germinate and plants can thrive.
A few examples of the soil temperatures fall-planted seeds prefer for germination: Snap bean 75-80, Beet 75, Cabbage family including chard and kale 68 to 75, Lettuce 65-70, Peas 65-70, Spinach 70, Winter Squash 75-80.
When soil temperatures are above those numbers, seeds remain heat-dormant. Some seeds will sprout when it is too hot or too cold but the percentage will be low (See http://tomclothier.hort.net/page11.html). The optimal temperature for seed germination is often provided in seed catalogs and printed on seed packets.
Days to maturity is also printed on seed packs and in catalogs. Since our average first frost day is November 15 for hard freeze (28-degrees F) and November 27 for killing freeze (24 degrees F). (See http://1.usa.gov/13Eaq7K most vegetables are harvested before those dates.
Days to maturity range widely and have to be considered when planning. For example: spinach and kale take 50 days, leaf lettuce is ready to harvest in 45 to 60 days but Brussels sprouts take 120 days.
Staying with the salad idea, greens planted this month can be harvested as micro-greens until the first killing frost in Nov. To harvest individual, outer, leaves, sow the seeds thinly. Plants that are tightly packed will provide more leaves for a shorter period of time. Wider plant spacing makes healthier plants.
Before planting, the soil has to be weeded, loosened, and amended with compost. Dampen the prepared row before sprinkling on the seeds since they are small and will be less likely to wash away if they stick. The row has to be kept damp with daily, light watering until the seedlings come up. Deep watering of maturing plants is better than sprinkling their tops.
Some gardeners prefer to start loose leaf lettuce plants in pots before planting the transplants in to a garden bed or container.
Spinach prefers to grow in the cooler temperatures of September but the seeds can be started in August. Spinach needs fertilizer, can survive 25 degree weather, and can be harvested within 45 days of planting.
An August-planted salad bed could contain lettuce, endive, beets, rocket, pak choi, mustard, peas, Red Russian kale, etc. Other vegetables that can be planted in August include bush beans (green or wax), cucumbers, summer squash, carrots, Brussels sprouts, collards, parsnips, potatoes, and chard.
Frost-tolerant vegetables can keep going past the first frost. They include beets, broccoli, cabbages, carrots, chard, cauliflower, and potatoes.
Tips: If you can give salad greens some afternoon shade days, they will thrive with less water. Mustard greens can be harvested 45-days after sowing, if the plants are kept consistently moist. Beets grow best when it is below 80 degrees.
If you want to plant potatoes, use seed potatoes. The main challenge is the risk that a killing frost will arrive before the potatoes form. Plant the potato pieces in a deep furrow and cover them with 2-inches of soil, then water and mulch with straw. From seed to harvest potatoes take about 110 days.
Remember to include friends and family, including children and seniors, in your garden planning. You can share seeds, planting, maintenance, and harvest.
Resources: OK State University Fact Sheet 6009, “Fall Gardening” http://bit.ly/18UVJSo and Burpee Seed Zone 7 fall planting guid http://bit.ly/19EJ60o.