Squash (Summer): Sow and Grow Guide

From long, green zucchini to yellow saucer-shaped patty pans, summer squash is extremely versatile in the kitchen-it can be sautéed, steamed, baked, or grilled. You can also add squash to omelets, soups, casseroles, lasagna, shish kebobs, sweet loaves (zucchini bread), or serve by itself. Summer squash's tender texture and buttery flavor is the stuff culinary dreams are made of. Luckily, they're pretty simple to grow! If you were wondering how to grow summer squash from seed, you've come to the right place!

General Information

Most summer squash plants grow in a bush form, and the fruits are harvested young when their outer skin is still soft, and seeds are immature. Summer squash's grown at home can store in the refrigerator for about a week and a half so enjoy shortly after harvest.

Did you Know?: Summer and winter squashes are closely related, and in some cases summer squash matures into winter squash. They have similar growing instructions and are afflicted by the same pests, too. They are, of course, different in growth and fruit. The word squash comes from the Narragansett Native American word, askútasquash, which means "eaten raw", even though these days we most often cook squash.

General Sowing

When to sow outside: RECOMMENDED. 2 to 4 weeks after your average last frost date, and when soil temperatures have risen to 70°–85°F.

How many do I plant? Summer squash are possibly the most productive plants in the summer garden. One plant generally produces enough fresh squash for 2 people to eat all summer. Summer squash can be cut into strips and used as a gluten-free, raw noodle substitute or pickled and canned, too. It may also aid in making friends with some neighbors by sharing the bounty.

When to start inside: Not recommended except in very short growing seasons, 2 to 4* weeks before transplanting. Use biodegradable pots that can be planted directly in the ground, minimizing root disturbance. *Be cautious to not mature seedling over 4 weeks as plants become stressed and potentially stunted.

Indoor Sowing

If you have a shorter growing season, you may consider starting your squash seeds indoors. Use a lightweight seed starting mix/media (sterile, and lighter than potting mix), and sow summer squash seeds 1/2"–1" deep, and winter squash 1" deep. Sow 2–3 seeds per pot, thinning to the strongest plant once leaves appear (clip extra plants at the soil level using scissors). The strongest plant may not be the tallest; look for thick, strong stems and deep color. By thinning early, you minimize the negative impact of crowding, like stretching for light.

Containers

Sow in 3"–4" biodegradable pots that can be planted directly into the ground, minimizing root disturbance. Biodegradable paperboard pots are the ideal size, easy to label, and convenient for sharing plants with friends. Squash roots are sensitive to transplanting, which is why direct sowing is our recommendation.

Transplanting

Harden off seedlings after 2 ½ to 3 weeks. Hardening off is the 7 to 10 day process of introducing pampered seedlings to the intense outdoor sun and temperature swings. Read our article on hardening off instructions.

Transplant into an area of full sun (6 or more hours a day), when soil temperature is at least 60°F. If you don't have a soil thermometer, this is often about 2 weeks after your last frost, but double-check the weather forecast. Transplant on a cloudy day or in the evening to reduce stress. Remove the bottom of the biodegradable pot when transplanting into the planting hole. This allows roots to escape easily while the remainder of the pot breaks down.

Outdoor Sowing

How to sow

Summer squash is commonly grown in mounds 3'–4' apart. Raised mounds warm the soil more quickly in spring and drain well. Vining summer squash can be grown on a trellis to save space, and increase airflow, which helps prevent fungal disease. If you choose to grow on a trellis, plants may be spaced more closely (half spacing).

Optimal Growing Conditions

Soil

Amend the soil ahead of time. We suggest submitting a soil test periodically, which gives you detailed information on your soil and how to improve it. Over-fertilization can invite pests, reduce fruit yield in favor of leaf growth, impact flavor, burn plants, or be a pollutant. Work soil so it is clump free, allowing it to drain well, and apply any additional fertilizer and/or organic matter needed. Initially, you just want the plants to grow, so usually a balanced fertilizer is best.

Water

It is normal for squash leaves to wilt in the heat of the day and recover overnight; it's their way of conserving water. To know if your plants need water, poke a finger into the mound, if you feel moist soil 2" down you can wait to water, but at about 3" go ahead and water. Avoid watering the leaves of the plant as it can encourage powdery mildew, a fungal disease that attack the plant leaves.

Exposure

Choose an area with full sun (6 or more hours per day).

Fertilization

Once your plants are almost ready to produce flowers and fruit, you can apply a phosphorous-rich liquid fertilizer to encourage blooming. Apply 4 weeks from transplant, or 6 weeks from seedling emergence if direct sown, repeating monthly. However, we always recommend a soil test to check for phosphorous levels first.

Weeding

Keep areas weed-free but cultivate shallowly as to not disturb shallow squash roots.

Pollination

Both summer and winter squash have pollen-producing ("male") and fruit-producing ('female") flowers on the same plant ("monoecious"). Bees are needed to transfer pollen from the pollen-producing flowers to the fruit-producing flowers to produce fruit. Pollen-producing flowers appear earlier in the season than fruit-producing flowers, in theory to attract bees to the area, so don't be alarmed if the first flowers do not produce fruit. Squash, as a native, has co-evolved with native "squash" bees. Squash bees are nest in the ground and you can create habitat for them by leaving a weed-free soil patch open and unmulched. Native squash bees are active earlier in the morning than the European honeybees and are also superior pollinators. Squash bees are focused on collecting pollen from all the flowers on the squash plants vs. the honeybee, which will collect pollen from many plants and may go to a few squash flowers and be distracted by another flowering species. Because squash bees are focused on squash, they ensure more fruit-producing flowers are pollinated completely, resulting in more and better-shaped fruit.

If you're not seeing any bees, sow bee-attracting flowers that bloom in early summer, like borage or alyssum, near squash plants. If bees continue to be few and far between, hand pollinating may be necessary. Use a paintbrush or remove one pollen-producing flower and press it into fruit-producing flowers (one pollen-producing flower can be used on up to 10 fruit-producing flowers).

Since squash are so dependent on bees, take extra precaution in applying any pesticides (organic or otherwise). Do not apply pesticides to flowers and apply very early in the morning or late in the evening when bees are less active; if you have native squash bees, evening may be best.

Harvesting

Blossoms

Look for pollen-producing, non-fruit producing flowers (fruit-producing flowers have a swollen mini-squash at the base of the flower and flowers are on shorter stems) and harvest just before use.

Harvest frequently to increase yield; squash seem to get monstrous overnight. While edible at almost any size, seeds are less developed in young fruit, therefore more tender. Using a knife or clippers, cut squash off including some of the stem. By including stem, the fruit is sealed and less likely to mold or dry out.

Common Pests and Diseases

Row covers at transplant or sowing can go a long way in pest prevention. You will not need to remove row covers until flowers appear, allowing plants to get a good, healthy head start. Scout unprotected plants for pests and disease weekly to catch any symptoms early. Attract native parasitic flies and wasps with small-flowered pollen and nectar plants like dill, cilantro, or alyssum.

Squash Vine Borers

Squash vine borer moths hatch out of their overwintering cocoons in the soil. These red bodied, wasp-like moths lay reddish-brown eggs around the stem close to the soil line. Deter moths by covering the base of the stems near the ground with soil, mulch, foil, or fabric material. The first sign of squash borers are eggs laid on the stem near the soil line, next to a pin-head sized hole, and sawdust-looking frass near the hole the borer made entering the stem; then part or all of the plant wilts. You can inject the stem with Bacillus thuringiensis (Bt) to kill the larvae or use a sanitized knife to cut above the entry hole, remove the larvae, and bury the damaged stem in hopes it will create new roots and recover. Because the caterpillars overwinter and rise again in the same fields, removing and destroying (not composting) infected plants is paramount in preventing ongoing borer infestations.

Squash bugs

Squash Bugs attack all squash. Both adults and nymphs suck plant juices, causing damage to the plant, which can result in a reduced harvest. Squash bugs' shiny bronze-colored eggs are most often found on the underside of the leaves, as a group of 10–20, in evenly spaced patterns. Squishing eggs is the most efficient way to eliminate infestation. Eggs hatch in about a week, making regular scouting and squishing necessary. Nymphs are pale green when young, becoming darker as they mature. Adults are fast crawling brownish black, flat-backed bugs. Protect young plants with floating row covers (remove covers prior to flowering to allow pollination, or hand-pollinate flowers.) Handpick all stages of squash bugs from the plant base and undersides of leaves and squish or put in a bucket of soapy water. Place boards on the ground around plants, and destroy adults found underneath every morning by scraping them into a pail of soapy water. Neem (an extract of the Indian Azadirachtaindica tree) has an odor that deters gray or brown adult squash bugs and if they eat it, it will disrupt their ability to reproduce (don't expect immediate knock-down). Insecticide soap and horticultural oil is effective with contact to young nymphs (adults are very difficult to kill) and will need to be reapplied to survivors or subsequent hatches. Kaolin clay products work to coat the leaf surface with a layer of clay creating a physical barrier for eating and egg-laying. Kaolin clay washes off with rain and will need to be reapplied. Also, sowing successions monthly, using row cover allows you to trap bugs and destroy infested plants starting fresh monthly.

Cucumber Beetles

Cucumber beetles looks like ladybugs but are yellow with black dots or yellow with black stripes. Both beetles spread disease like bacterial wilt and should be controlled as soon as possible. Kaolin clay products create a layer of clay that makes feeding on the plants difficult for the cucumber beetles. Spinosad is a soilborne bacterium that is the active ingredient in other organic cucumber beetle controls. Straw mulch makes beetle movement more difficult and creates habitat for wolf spiders, which prey on cucumber beetles. Intercropping broccoli, nasturtium, or radish with squash has been shown to reduce damage from cucumber beetles.

Powdery Mildew

Powdery Mildew, one of the more common fungal diseases in the garden, will first appear as powdery white or gray spots which rapidly spread to cover leaves, tender stem tissues and flower buds. Infected leaves often shrivel and dry, which exposes fruit to sunburn, and may cause small fruit size and poor texture. Powdery mildew thrives in humid conditions, especially with cool nights. Healthy plants are less susceptible to infection by powdery mildew fungi, so keep plants properly fertilized and watered, and thin plants to improve air circulation around leaves and stems. Avoid overhead watering of foliage, and water early in the day so any wet leaves will be quickly dried by the sun. Thorough fall clean-up of plant debris will remove overwintering sites for fungi. Do not compost infected plant material. Organic controls include copper, sulfur, compost tea, horticultural oil plus baking soda, milk, and neem-based products. Try a spray of compost tea or baking soda plus horticultural oil, spraying plants thoroughly and frequently every 7 to 10 days to prevent the spread of the disease. Compost Tea, long used as a fertilizer, may also help fight fungal diseases. Begin with a burlap or cheesecloth bag containing 1 gallon of well-aged, manure-based compost. Place in a 5-gallon bucket of water, stir well, and steep it in a warm place for 3 days. Then remove the bag, put the liquid in a sprayer or watering can, and spray or sprinkle the entire plant. Oil Spray combined with baking soda has been found to prevent powdery mildew, also. Mix 1 tbsp. of baking soda and 2 tsp. of horticultural oil in 1 gallon of water, and spray plant thoroughly once a week. Be sure to follow the application directions on the oil label; if the plant you are treating is not listed on the label, test the spray on a few leaves first, waiting 48 hours to make sure there is no spotting or discoloration. Some studies have shown that milk can be as effective as commercial fungicides, suggesting that a 10% solution is effective if used several times a week or 20–50% was needed if used weekly. Dilute milk to the desired percent in water, and add a couple drops of natural soap to help the spray stick to leaves, rather than rolling off.