Growing Summer Squash (Including Zucchini) in the California Home Garden

Provided that a few important cultural needs are met, summer squash and zucchini can be among the most rewarding of garden vegetables. When harvested religiously while fruits are small, zucchini can be so productive over such a long period of time that when faced with yet another round of new fruits for harvest, some gardeners may wish that they had made more mistakes in cultivation.

Under adverse conditions or when their needs aren’t met, summer squash and zucchini plants may grow slowly or become stunted, produce little or no fruit, or become prone to pests and diseases. GardenZeus expert Darren Butler offers the following tips for growing summer squash and zucchini (note that for purposes of this article, unless specified otherwise, all discussion of summer squash includes zucchini):

Cultural needs and environmental conditions: Summer squash is a warm-season plant that likes cool weather but is harmed or killed by frost. They grow and produce best at daytime temperatures of about 65° to about 75°F, and while many varieties tolerate heat, growth and fruiting may be diminished at temperatures above 85°F. Time your plantings to coincide with cool-but-not-cold weather.

Summer squash performs best in full sun, with ample growing space that varies by variety; and in rich, loose, fertile, well-drained soil that is consistently moist but not wet. Squash is a heavy feeder that needs sufficient nutrients to grow quickly and produce large yields. It suffers in compacted, uncultivated, or infertile soils. Avoid planting squash repeatedly in the same soil or in soils exhausted by other heavy feeders.

Planning and preparation: Amend soils with generous proportions of composted manure and nutrient-rich compost. Single-dig or double-dig soil to loosen it to a depth of at least 12 inches, remove stones and obstructions, and add compost and composted manure shortly before planting squash.

In gardens with new, uncultivated, infertile, or compacted soils, plant pre-germinated seeds directly into mounded hills, raised beds, or large containers with sufficient space. Use a mixture of washed sand, topsoil, and up to 20% composted manure and nutrient-rich compost in the beds.

When growing vining varieties, whether in raised beds, containers, or in the ground, it usually works best to install trellises or other support before planting seeds. Generally the longer you wait to install support after planting squash, the more awkward and unworkable it may become. Consider also installing drip irrigation before planting, which can save hassle and damage to plants later on. With drip tubing already in place, it’s easy to plant seeds or pre-germinated seeds near individual drip emitters. Tubing may need to be adjusted or moved from time to time as plants establish and eventually become large.

Germination and planting: Plant seeds, not seedlings. Most cucurbits are sensitive to any root disturbance, such as from transplanting. Surface cultivation, digging, harvesting root crops, thinning, or weeding near established plants may also cause root disturbance to cucurbits. From the time of germination onward, it’s generally best to avoid all root disturbance as much as possible for all cucurbits.

Summer squash should generally be seeded directly outdoors, when daytime temperatures are at least 65°F and frost risk is low. Seeds may be slow to germinate or rot before germinating at temperatures below 65° F, and germinate best at nighttime temperatures above 55°F. GardenZeus recommends against purchasing squash starts for transplant, especially if rootbound or it has been more than about 2 weeks since germination.

Summer squash germinates best at warmer temperatures of 70° to 95° F, with fastest germination at warmer end of this range. Seeds should be kept moist but not wet for germination. Generally, summer squash should germinate within a week if kept moist at temperatures of 70° to 95°F. If more than a week has passed under these conditions, investigate your seeds. Something may have gone wrong or you may need to replant.

While transplanting squash is tricky, it’s easy to germinate seeds indoors for immediate planting outdoors. Space seeds widely on a moist paper towel in a covered glass dish. Monitor seeds daily and plant directly outdoors as soon as a root emerges. Germination is faster with bottom heat, such as from an appliance or germination heating mat or pad (do not germinate seeds with heating pads meant for human use, which are not designed to accommodate moisture or use for long periods). Use care when handling tiny, delicate roots. If roots have grown into the paper towel, tear it carefully and plant the germinated seed with paper towel attached.

Plant summer-squash seeds at a depth of about 1/2-to-1 inch in moist or pre-irrigated soil. As a general rule, wait to water germinating squash seeds again until the soil surface is dry down to about 1/8 to 1/4 inch.

Many summer squash and zucchini varieties are traditionally planted with 2 to 4 seeds per planting hole, spaced about 2.5 to 3.5 feet apart, then thinned to the strongest plant. To thin zucchini, snip off unwanted seedlings at soil level when they are about 2 to 3 inches tall without disturbing the roots of the remaining seedlings.

Squash plants need good drainage and may benefit from being planted in mounds that are raised several inches to a foot or more above grade, and often about 18 inches in diameter, with hills spaced 3 to 4 feet apart. Mounding is particularly recommended when getting an early start on the season, planting during cool-to-cold weather, or for skilled gardeners attempting to grow zucchini during cool weather or in mild-winter areas of California from fall through spring, as mounded soil is often warmer than soil below grade. Plant a few to several seeds around the sides of the mound and thin to 1 or 2 plants per mound.

In loose and sandy soils, squash may benefit from being planted in a shallow hollow or depression with mulch to retain soil moisture. The hollow can be filled to provide the extra water squash may need in sandy soils while avoiding runoff. Keep mulch a few inches away from squash stems to avoid encouraging rot.

If you’ve had trouble with low yields from summer squash or zucchini, try planting directly into a mature compost pile. If planning well in advance, construct a compost pile inside a wire cage that is about 2-to-3-feet wide and about 3-to-4-feet tall beginning in fall. With sufficiently small openings in wire mesh and closure at the top and bottom, this can also help to eliminate rodent and mammal-pest activity in compost piles. Refresh the compost pile all winter with new organic matter. Once you have a layer of rich compost at least a foot or two high, ideally well-populated by earthworms, plant summer squash or zucchini directly into the cage or just outside the cage in late winter or spring (depending on your growing season). The wire cage will then provide trellising or support for vining plants. Nutrient-rich soil in and under compost piles will help to produce vigorous, healthy squash plants that yield abundantly if their cultural needs are otherwise met.

Summer squash can be effectively transplanted within about 10 to 18 days after germination; this is recommended for skilled biointensive gardeners and experienced gardeners who are willing to monitor seedlings closely. GardenZeus recommends germinating squash in pots of at least 4 inches width and depth. Squash starts should be transplanted in the short window after germination and before their roots encounter the sides or bottoms of cells or pots, generally before they develop a second set of true leaves. If you miss your transplant window, it may be best to start over, depending upon the degree of root binding of seedlings. Handle seedlings carefully and minimize disturbance to rootballs. Even vigorous, healthy, young squash starts can be shocked due to root disturbance while transplanting. Plant slightly deeper than a seedling was in a cell or pot to protect its root crown.

Care and maintenance: Maintain consistent soil moisture while keeping squash leaves entirely dry. Avoid watering with sprinklers. Prevent wilting. Walking or working near squash plants when they are wet after rain or from overhead irrigation may encourage the spread of foliar diseases. See “Watering Tips” in the GardenZeus section “Planting and Maintaining” for more information. See also The GardenZeus Guide to Watering Cucumbers, Melons, and Squash (Cucurbits).

Provide sufficient nitrogen throughout the growing and harvest periods. Squash needs nitrogen to grow quickly and yield abundantly. See “Nutrient/Fertilization Tips” in the GardenZeus section “Planting and Maintaining” for more information.

Harvesting: Harvest all zucchini fruits when young, at about 7 inches maximum length. When zucchini fruits are allowed to grow large and ripen on the plant, this exhausts the plant. See “Harvesting Tips” in the GardenZeus section “Seasonal and Harvesting” for more information.