How to Grow Zucchini

Planting

Zucchini is a summer squash, a bushy form of Cucurbita pepo that is eaten young. It needs full sun, fertile, well-drained soil and warmth. Clemson notes that squash seed germinates poorly in cold soil and waits for 60°F at 4 inches deep; Utah State plants at 65°F. That usually means 1 to 3 weeks after the last frost, as the dates for your zone above show.

Sow seeds 1 inch deep in hills. Iowa State plants 4 to 5 seeds per hill and thins to 2 or 3 strong plants once they have one or two true leaves; Utah State thins to two. Space hills and rows 3 to 4 feet apart (Iowa State). In rows, Clemson thins seedlings to 12 to 15 inches apart with rows 3 feet apart.

Transplants give an earlier harvest in short-season areas. Start them 3 to 4 weeks ahead in peat pots or other plantable containers, since squash roots do not tolerate disturbance, and harden them off before planting. With black plastic mulch and row covers, Utah State plants 2 to 3 weeks before the last frost date.

Fertilizer and water

Without a soil test, Iowa State works 1 to 2 pounds of 10-10-10 into each 100 square feet before planting; Clemson uses 3 pounds of 5-10-10 over the same area. When the plants start to run, side-dress with nitrogen: Utah State uses 3 to 4 tablespoons of 21-0-0 per plant, watered in, and Clemson 1 pound of 34-0-0 per 100 feet of row.

Water deeply, 1 to 2 inches a week, preferably by drip so the leaves stay dry. Clemson says moisture matters most during fruit set and development and should reach 6 inches deep. Uneven watering and too much nitrogen both interfere with calcium uptake and lead to blossom-end rot, where the flower end of the fruit rots. Iowa State says the cure is steady moisture, not added calcium. Hold off on organic mulch until the soil reaches about 75°F.

Flowers and pollination

Squash plants carry separate male and female flowers. The first flowers are male and drop without fruit, which is normal. Female flowers have a small zucchini at the base and appear soon after. Bees must move pollen from male to female flowers; if they don’t, the young fruit starts to grow, then yellows and shrivels. Iowa State notes that bees and other pollinators are less active in cold, rainy or windy weather. Avoid daytime insecticide sprays that kill pollinators.

Varieties

Extension services recommend these zucchini varieties for their regions:

• Southeast (Clemson): ‘Black Beauty’, ‘Eight Ball’, ‘Elite’, ‘Embassy’, ‘Senator’ and ‘Spineless Beauty’. ‘Black Beauty’ dates from the 1920s.
• Midwest (Iowa State): ‘Elite’, ‘Jaguar’, ‘Spineless Beauty’ and the golden zucchini ‘Goldfinger’.
• Florida (UF/IFAS): ‘Cocozelle’, ‘Spineless Beauty’ and ‘Black Beauty’.

Utah State says most zucchini varieties perform well throughout Utah.

Regional notes

Cold North. Zucchini’s short season makes it one of the most dependable warm-season crops in the North. Colorado State notes that summer squash, like cucumbers, does well above 5,000 feet because it matures quickly.

Humid South. The season is long enough for two crops. Clemson plants from mid-March or mid-April into July and again in late summer: July 15 to August 15 in the Piedmont and through August on the coast. Powdery mildew and aphid-spread viruses are the main diseases Clemson lists.

Desert Southwest. In the Phoenix area, the University of Arizona sows summer squash from mid-February to early April and again from mid-August to early September, avoiding the hottest weeks.

Warm-winter zones 9 to 11. UF/IFAS grows summer squash through the cool season in South Florida, from August to March, and in spring and late summer farther north.

Pests and diseases

Squash vine borer causes the sudden midsummer collapse that ends many zucchini plantings. The larvae of a day-flying, wasp-like orange and black moth bore into the stem near the soil, and the plant collapses. UConn notes that C. pepo, including zucchini, is highly susceptible because of its thin, hollow stems. Defenses:

• Cover plants with row covers by early June and remove them when flowers open, but do not use covers where squash grew last year, since adults emerge from the soil beneath (Iowa State).
• Wrap foil or nylon around the base of young stems before moths appear (UConn).
• Plant a second crop in late June or early July, after egg-laying ends. Iowa State says this works for fast-maturing zucchini, and it also replaces tired spring plants.
• If borers get in, slit the stem, remove the larvae, and bury the cut section so it can root (UConn). Insecticides only work before larvae enter the stem.

Squash bugs suck sap until leaves wilt. Crush the bronze to brick-red egg clusters under the leaves, hand-pick adults, and trap them under boards laid near the plants, checking each morning.

Powdery mildew coats leaves with white powder from midsummer. Give plants full sun and good spacing, avoid overhead watering, and grow resistant varieties where available.

Cucumber beetles and aphids spread bacterial wilt and viruses. Row covers on young plants help with both.

Remove and destroy vines after harvest to cut down overwintering squash bugs and borers.

Harvest

Zucchini grows fast, and the fruit is usually ready 3 to 5 days after the flower opens. Pick at 6 to 8 inches long and about 2 inches thick (Clemson suggests 7 to 8 inches), while the skin is glossy and soft enough to dent with a fingernail and the seeds are small. Iowa State allows long types up to 12 inches, but quality drops as the skin toughens and seeds form. Check plants every day or two, and pick oversized fruit too so the plant keeps setting new ones.

Handle the fruit gently, as it bruises easily. Store zucchini in a plastic bag in the refrigerator crisper, where Iowa State says it keeps 5 to 7 days. Extra fruit can be frozen or pickled.