What to Plant in August for a Fall Vegetable Harvest

Vegetables to Plant in August

From the upper Midwest to New England and the mid-Atlantic, August is the month to sow leaf lettuce, spinach, radishes, turnips, green onions and kohlrabi, and to finish planting kale and broccoli, for harvest in September, October and November. Slower crops such as carrots, beets and cabbage usually need a July start in the North. Bean, cucumber and squash plantings in August only pay off where frost is still months away.

Iowa State University Extension publishes fall planting windows that work as a benchmark for the upper Midwest. These crops have a window that includes August.

Crop	Days to maturity	Planting window in Iowa
Kale	55 to 75	Mid-July to early August
Broccoli (transplants)	60 to 65	Late June to early August
Kohlrabi	55 to 70	Mid-July to mid-August
Leaf lettuce	45 to 60	Early August to early September
Turnips	45 to 70	August
Winter radish (daikon)	45 to 70	August
Spinach	45 to 60	Mid-August to early September
Radish	25 to 40	Mid-August to mid-September
Green onions (plants or sets)	30 to 50	Mid-August to mid-September

Gardeners with a later frost than Iowa can shift every window later, and those farther north should shift earlier. The formula below does that adjustment for any frost date.

What Other Regions Plant in August

In New Hampshire, the University of New Hampshire Extension lists a long August menu: beets, broccoli, bunching onions, carrots, cauliflower, cilantro, collards, daikon, kale, kohlrabi, leaf and head lettuce, mustard greens, peas, radishes, spinach, Swiss chard and turnips. The first freeze there comes in early to mid October in the central and southern parts of the state and mid to late September in the north, so the slower crops on that list belong at the start of the month.

In Maryland the season runs later. University of Maryland Extension notes that looseleaf lettuce, radish, arugula, mizuna and tatsoi grow quickly and can be planted several times from late August through late September for a continuous harvest. Sowing a short row every week or two is the same method described in the guide to succession planting lettuce.

Count Back From Your Frost Date

Every fall planting date comes from one calculation. Look up the average first fall frost for your location from your state extension service, then count backward.

1. Start with the date you want to begin harvesting. For hardy crops this can be the frost date or later. For tender crops it must be earlier.
2. Subtract the days to maturity printed on the seed packet.
3. Subtract 14 more days. University of Maryland Extension calls this the Short-Day Factor, and Virginia Cooperative Extension calls it the Fall Factor. It accounts for slower growth as days shorten and cool.
4. For frost-tender crops, subtract another two weeks. Virginia’s Frost-Tender Factor exists because corn, beans, cucumbers, tomatoes and squash have to mature two weeks before frost to give a harvest.

Maryland works an example. For an October 15 harvest of beets with 55 days to maturity, 55 days back is August 21, and 14 more days back gives a planting date around August 7. By the same arithmetic, a 45-day spinach aimed at October 15 goes in around August 17, a 60-day kale around August 2, and a 30-day radish around September 1.

Two adjustments keep the math honest. New Hampshire’s extension service points out that days to maturity are estimates counted from germination, so seed that takes a week to come up in dry soil is a week behind. For transplants, Virginia’s version of the formula counts the days from seed to transplant separately from the days from transplant to harvest, and adds the length of the harvest period for crops picked over several weeks.

How Much Frost Each Crop Can Take

The harvest date in step one depends on how the crop handles cold. Iowa State sorts fall vegetables into three groups.

Group	Tolerates	Crops (Iowa State University Extension)
Tender	No frost	Green beans, corn, cucumber, okra, summer squash
Semi-hardy	Down to the upper 20s°F	Beets, carrots, leaf lettuce, green onions, kohlrabi, radish, winter radish, spinach, Swiss chard
Hardy	Down to the mid to lower 20s°F	Broccoli, Brussels sprouts, cabbage, cauliflower, kale, rutabagas, turnips

Harvest of the semi-hardy group often happens after the first frost date, and the hardy group can stand in the garden later still. That is why kale and turnips are safer August bets than their days to maturity suggest. Virginia Cooperative Extension adds that the flavor of cool-season crops such as cabbage, cauliflower, broccoli, spinach and Brussels sprouts may be enhanced after a frost.

Sources group a few crops differently. Maryland places lettuce and spinach in its hardy group and cauliflower in its half-hardy group, so treat the borderline crops with some caution and keep a row cover handy.

Tender Crops: Usually Too Late

Beans, cucumbers and summer squash need the longest lead of any fall crop because frost ends them outright. Iowa State’s table has green beans at 50 to 70 days with a planting date of mid-July. Under the formula, a 50-day bean aimed at an October 15 frost needs 50 days, plus 14 for the fall factor, plus two weeks for the frost-tender factor, which lands on July 29.

The same 78 days counted from August 1 reach October 18, and from August 15 they reach November 1. An August sowing of tender crops therefore suits only places where the first frost comes in late October or later. Late squash can also meet late pests, and the page on squash vine borer prevention lists the states where a second generation of borers is possible.

Getting Seeds Up in August Heat

Germination is the hard part of August planting. University of Maryland Extension says lettuce and spinach seeds are difficult to germinate when the soil temperature is above 70°F, and advises waiting until summer temperatures cool. Iowa’s spinach window, which opens in mid-August, fits the same advice.

Extension services offer several ways to cool and dampen a seed row:

• Lay a board over the row after sowing and watering, and remove it when sprouts just reach the soil surface (Virginia).
• Put shade cloth over the bed to reduce temperature and improve germination (New Hampshire).
• Spread a light mulch to cool the soil (Iowa State).
• Water as often as needed, which may mean multiple times a day, to keep the top inch of soil evenly moist until seedlings are established (New Hampshire).
• Pre-sprout seeds indoors between moist paper towels and plant them once they germinate (Iowa State).

Transplants sidestep the problem. Broccoli, cabbage and cauliflower appear in Iowa State’s table as transplants only. If you are clearing a spent summer crop to make room, the comparison of hot and cold composting covers what to do with the old plants.

Daylight Sets the Final Deadline

Cold is one limit and light is the other. University of Maryland Extension notes that vegetable growth is very limited when sunlight drops below 10 hours a day, which in Maryland runs from about mid-November through late January. Farther north the 10-hour mark arrives earlier in the fall.

Plan for fall crops to reach nearly full size before that date. After it, hardy plants hold in the garden and can be picked as needed, but they add little new growth. Maryland suggests row covers, cold frames and low tunnels covered with plastic film to protect plants and extend the harvest. Our guide to cold frames and row cover types compares them.