Fall Planting Guide: Count Back From Frost

Overview: five steps from frost date to sowing date

A fall garden is planned backward. You start at the day frost usually arrives and subtract.

Step 1: Find your average first frost date

Look up the average date of your first killing frost. Your county extension office publishes it, and NOAA climate normals list it by weather station.

The date moves a lot between regions. Iowa State says the first frost in Iowa arrives between September 30 and October 15. A University of Illinois Extension post puts it in mid-October for central Illinois. Your own date can be weeks earlier or later.

Treat the date as an average. Virginia Cooperative Extension says actual first-frost dates vary from year to year. It also warns that a low, unsheltered spot is likely to be a frost pocket, so a bed at the bottom of a slope may freeze before the rest of the yard.

The frost date does not move, so fall planting is a subtraction problem.

Step 2: Add up the days for each crop

Take the crop's days to maturity from the seed packet. Then add the time the fall will steal from you.

Virginia Cooperative Extension gives the sum. Count back from the first frost by the days to harvest, plus a Fall Factor of about two weeks. Days get shorter and nights get cooler, so plants grow more slowly than in spring. Add another two weeks for frost-tender crops. Virginia names corn, bean, cucumber, tomato and squash as those crops, because they must finish two weeks before frost to give a reasonable harvest. The Illinois post agrees and says to add a week or two for the fall factor.

Use the long end of the range on the packet. A late sowing has no spare days. Virginia also says to choose the earliest maturing varieties for late plantings.

Step 3: Pick crops from the fall planting guide table

The table below runs the sum for a first frost on October 15. Days to maturity are the long end of the ranges in the Iowa State guide. Iowa State lists broccoli as transplants, so its count starts when you set the plant out. The dates are my arithmetic, not a state calendar.

Bush beans need 98 days and radishes need 54. That is a 44-day gap, so the two crops go into the ground more than six weeks apart for the same frost. Sow the beans by early July. The radishes can wait until late August.

If you want the same sum on paper, The Winter Plan's fall countback worksheet walks through a worked example.

Step 4: Sow into warm, dry midsummer soil

Midsummer soil is often dry, and dry soil can keep seeds from sprouting. Water the bed well the day before you sow, or sow after a rain.

Sow the seed about twice as deep as you would in spring. An old trick, Virginia notes, is to water the seeds in and lay a board over the row until the sprouts reach the soil surface. Lift the board then so the seedlings get light. A thin layer of organic mulch keeps the soil cool and moist. Illinois also suggests a light covering of mulch or a board.

Broccoli and cauliflower go in best as transplants. The Illinois post says to start the seed indoors 6 to 8 weeks before you set the plants out. Count that into your dates. A 6-week start for broccoli sown by July 28 means seed goes into trays around mid-June.

Restore what the summer crop took out. Virginia suggests a light layer of compost or aged manure before fall crops go in. Plant a different crop family than the one that grew there in spring, to avoid carrying pests over.

Step 5: Watch the first frost and harvest

Frost splits your garden in two. Tomato, summer squash, melon, eggplant, cucumber, pepper and okra cannot stand it. Pick them as soon as frost comes, Virginia advises. If the warning is mild, no lower than 30 degrees F, covering the plants with baskets, burlap, boxes or blankets can protect them.

Cool-season crops can wait. Cabbage, cauliflower, broccoli, spinach and Brussels sprouts withstand some cold, and their flavor may be better after a frost. Illinois puts the lines at 32 degrees F for semi-hardy crops such as beets, carrots, cauliflower and lettuce, and 28 degrees F for hardy ones such as broccoli, cabbage, radishes and spinach.

Some crops can stay in the ground much longer. Kale, spinach, lettuce, parsley, parsnip and carrots may survive all winter in the garden, Virginia says, and it recommends 8 inches of mulch over them to prevent heaving.

Fall sowing for next summer: garlic

Garlic is planted in fall and harvested the next summer, so it runs on a different count. Penn State Extension says fall is the best time to plant garlic, a good three weeks before the ground freezes, so roots form but do not poke through the surface before winter. Set the cloves root side down, at least 2 inches below the surface, and mulch with a good 4 inches of straw or shredded leaves. A University of Illinois Extension post puts mid-October at about the right time for its area and sets cloves pointy end up, 1 to 2 inches deep, with harvest in mid-summer the following year. Both give a ground-freezing or area cue rather than one date, so count back from your own frost.

When your state's free dates are all you need

If your state extension already publishes a dated fall planting chart for your hardiness zone, use it. Virginia, for one, points readers to a planting-dates guide, Publication 426-331. A chart like that does the countback for you, and a kit adds nothing for a gardener who has one that fits.

A printable kit suits the gardener who wants every crop's sum worked on one page. The Winter Plan counts from your own first-frost date and carries a 108-station freeze table, so it fits any zone. You fill in the countback worksheet in midsummer, tick off the fall countdown through August and September, and keep the result on the notes for next winter page.