Winter Squash Planting Season: Count Back From Frost

Soil temperature opens the winter squash planting season

Winter squash is a very tender warm-season plant. The University of Maryland Extension says frost will injure top growth and that the plant needs warm weather to grow. Its home garden fact sheet says to plant after all danger of frost has passed and the soil has warmed.

Three other universities put a number on "warmed." Penn State says soil should be at least 65 degrees F for good germination. South Dakota State says to plant after the chance of frost has passed or when soil reaches 70 degrees F. A 2021 Georgia extension bulletin gives an ideal soil range of 65 to 80 degrees F.

So the opening day is not your last frost date. It is the first day after that date when a soil thermometer reads 65 to 70 degrees F at planting depth. In a cold spring that day can come weeks late.

Winter squash needs a warm start and a long, frost-free finish.

Days to maturity differ by source, so use your packet

Each source gives a different range for how long a vine needs. The range is wide because a small acorn type and a large banana type are not the same plant. The seed packet for your variety beats any general number.

Treat the low end of each range as a fast variety and the high end as a slow one. If your packet says 95 days, use 95 in the sums below.

Count back from your first frost to find the last sowing day

The season closes at the other end, and the close is easy to miss. Maryland and Penn State both say winter squash is harvested in September or October, before the first hard frost. A vine that has not ripened its fruit by then is out of time.

Here is the sum with example dates. Say your last spring frost is May 10 and your first fall frost is October 20. That gives 163 days between them. A 100-day squash sown on the last-frost day would use 100 of those days and leave 63 days of slack. The last day you can sow it is July 12.

The slack is a number you can plan around. Every day you wait for warm soil, or for a late-spring rain, comes out of it. Treat the day count as starting on the sowing day, which is the cautious way to read the packet.

Replace October 20 with your own first fall frost and subtract again. The Maryland sheet says you can still plant a second crop between June 15 and July 1. A 110-day type sown on those dates would ripen between October 3 and October 19, so that window only suits a late first frost.

Can you plant winter squash in September or October?

Usually no. Run the table the other way. A 60-day squash sown on September 15 needs a first frost after November 14, and a 100-day type needs one after December 24. Only a garden with a first frost that late can use those dates.

Soil is the second test. Penn State puts the floor for germination at 65 degrees F, so check with a thermometer before you sow. If your first frost is close, the practical answer is to order seed now and sow after your next last frost.

Where the frost-free season is long, the sum can work. Count forward from today by the days on your packet and check the soil with a thermometer. If the result lands before your first frost, you have room. If it lands after, wait.

Start indoors or sow in the ground

Maryland says you can direct sow 2 to 3 seeds per spot, or start transplants indoors in 3-inch containers about 3 weeks before planting time. Thin the seedlings to the strongest one by pinching or cutting the extras, not pulling them. Handle transplants gently and disturb the roots as little as possible.

Indoor starts can buy you some of the slack from the sum above. Wait for the same 65 to 70 degrees F soil reading before you set them out, and check it with a thermometer.

Sowing depth varies by source. Maryland says 4 times the thickness of the seed, Penn State says 1 inch, and South Dakota State says about half an inch, so the packet is again the tiebreaker. Squash wants well-drained, fertile soil, per Penn State, with a pH between 6.0 and 6.8, and prefers sandy loam or raised beds over heavy clay. It wants at least 6 to 8 hours of direct sun a day, and Maryland says the crop prefers 8 to 10.

Spacing is wider than most gardeners expect. Maryland says to plant every 3 to 4 feet in rows 6 to 8 feet apart, depending on the type. In a raised bed, keep each vine at least 3 feet from the next. Maryland adds that small-fruited types can climb a trellis if you support the fruit. A dated plan that shows the true bed size makes the choice easy, and The Winter Plan prints its bed grids at 1 inch to 1 foot for that purpose.

Mistakes that leave squash unripe at first frost

Five slips come straight from the numbers above. Each one eats the slack.

• Sowing into soil under 65 degrees F. That is the floor for good germination, says Penn State, so seed sits and the clock runs.
• Trusting a general day count. Sources range from 60 to 120 days, so use the figure on your own packet.
• Sowing a long-season type late. Maryland allows a second crop from June 15 to July 1, which only fits a short variety or a late first frost.
• Waiting past the sow-by date from the sum above, even by a week.
• Leaving fruit in a hard frost. Temperatures below 28 degrees F weaken the rind, according to Penn State, and let bacteria in, which shortens storage. The same page says pumpkins that have begun turning orange can finish ripening indoors.

South Dakota State gives a ripeness test, which is a rind too hard to pierce with a fingernail, plus a corky stem.

What a row gives you, and what to spray

Extension sheets count squash by the row, not the plant. Maryland gives an approximate yield of 10 to 20 pounds per 10-foot row. The sources used here do not give a fruit count per vine, so plan by row length and weigh your first harvest.

Maryland lists powdery mildew among the problems of winter squash. For a home remedy, South Dakota State suggests 2 tablespoons baking soda and 1 teaspoon cooking or horticultural oil in 1 gallon of water. A University of Wisconsin fact sheet says a mix of 1½ tablespoons baking soda and 3 tablespoons of light horticultural oil in 1 gallon of water has been shown to be effective. It advises spraying every 7 to 14 days and testing a small area first.

South Dakota State also notes that diseases are more common when squash or pumpkins grow in the same spot more than one year. Move the vines each season, and write down where they were.

One or two vines need only a sticky note

If you grow one or two vines, a sticky note is enough. Subtract the days from your first frost, write the date, and you are done. Free extension sheets cover the rest, and a local one written for your climate beats a general count.

A printed plan pays off when squash is one of many crops. The Winter Plan counts from the frost dates you enter, so one set of sums covers any US zone. You run the worksheet on sowing day with your packet in hand, and the fall countdown page tracks the weeks until you cut the vines.