When to start squash and zucchini seeds

Why direct sowing is preferred

University of Minnesota Extension says direct seeding is the preferred way to start squash. Squash seed won't germinate in cold soil, plants started indoors and set into cold soil don't grow well, and transplants are often set back by root damage. Sow after the last frost date once the soil is at least 70°F at a two-inch depth. University of New Hampshire Extension uses the same 70°F rule for summer squash, zucchini, winter squash and pumpkins. In most of Minnesota, UMN says, that is late May, or early June in the north.

How to sow

Sow about half an inch deep (UMN). For vining types, sow 2 inches apart in a row, allow 2 to 3 feet on each side for the vines, and thin to 8 to 12 inches apart. Or plant hills of three or four seeds with 5 to 6 feet between hills. Bush types fit closer, with 2 to 3 feet between rows or hills.

For a longer, steadier zucchini harvest, UMN suggests sowing only part of your squash at first and the rest three weeks later.

If you do start indoors

UNH gives 3 to 4 weeks from seed to transplant, setting out 2 weeks after the last frost. With an average last frost of May 10, you would sow indoors April 26 to May 3 and set plants out May 24. Use pots you can plant whole and keep the root ball intact. Small, young transplants handle the move better than large ones.

Germination

Harrington's data (published by Oregon State Extension) give squash a 60°F minimum, a 70–95°F optimum range and a 95°F optimum. Iowa State Extension lists 75–80°F with seedlings in 5 to 10 days, and notes squash and pumpkin are often direct sown once temperatures are warm enough.

Warming the soil and pollination

Black plastic mulch, laid after you prepare the soil, absorbs heat and lets you plant earlier (UMN). Spun row covers over seedlings protect from cold nights, but they also keep out bees. Squash has separate male and female flowers and needs insects to carry pollen between them, so remove covers once flowering begins.

Winter squash timing

Winter squash and pumpkins follow the same planting rule, but they need many more days to mature. Check the days to maturity on the packet against your first fall frost; the fall planting calculator gives the latest safe sowing date for frost-tender crops like these.

Common mistakes

• Planting on the frost date. Wait for 70°F soil, which can come a week or more later.
• Starting indoors too early. Root-bound seedlings stall after transplanting.
• Planting everything at once. Stagger sowings three weeks apart for a steadier harvest.
• Covers left on at flowering. No bees means no fruit.