Growing zucchini and summer squash in Minnesota home gardensVincent A. Fritz, Extension Horticulturist, Southern Research and Outreach Center
Carl J. Rosen, Extension Soil Scientist, Department of Soil, Water, and Climate
Reviewed 2009 by Cindy Tong, Extension Horticulturist
Summer squash (Cucurbita pepo), including zucchini, crookneck, straightneck, patty pan, and other similar types, is grown in many, if not most, Minnesota vegetable gardens. Squash fruits are eaten cooked, raw, and shredded or grated in baked goods. Squash flowers are edible, as well.
Like other “vine crops,” summer squash plants grow most vigorously and produce the most fruit in warm weather. Some varieties form long, rambling vines; others are bush types that fit more easily into a small garden.
Squash plants bear separate male and female flowers (monoecious flowering habit), and pollen must be transferred from the male flowers to the female flowers by insects. Male flowers are attached to the plant by a slender stem. Female flowers are attached close to the main vine, and between the flower and the vine is a small round ovary, the unfertilized fruit. Squash flowers are typically pollinated by bees.
Squash seeds will not germinate in cold soil, nor will plants started indoors and set out into cold soil grow very well. Further, the taproot of squash and other vine crops must not be damaged in transplanting, so plants started indoors often fail to thrive in the garden. Direct-seeding is the preferred method for starting squash. Use a soil thermometer and sow seeds after the last frost date, once soil has warmed to 70° F at the 2-inch depth. In most of Minnesota, this will be sometime in late May.
Earlier planting is possible with the use of black plastic mulch, which raises soil temperature, along with spun row covers, which raise air temperature around the plants and protect them from cold nights. Apply black mulch once the soil has been prepared in spring. The black color will absorb heat from the sun and warm the soil faster than if it were uncovered. Cut holes or slits in the mulch, and plant the seeds as above. After the seedlings have emerged, position the row covers over the plants, securing the edges with soil or staples. Row covers exclude both pests and beneficial insects required to pollinate the flowers, so they must be removed from the plants once flowering begins.
Some varieties of summer squash produce fruit over a longer season; some set lots of fruit within a shorter period. It may be worthwhile to seed only part of your squash garden at first, then finish the planting three weeks later, to ensure a longer, more even harvest.
Sow seeds about ½ inch deep. For vining types that will be allowed to spread out in the garden, seeds should be sown 2 inches apart. Allow about 2 or 3 feet of space on either side of the row for the vines to spread. After emergence, thin seedlings to stand 8 to 12 inches apart. A “hill” of 3 or 4 seeds sown close together is another way to plant squash in the garden. Allow 5 to 6 feet between hills.
Bush types, with very short vines, can be planted in closely-spaced hills, or in closely-spaced rows, with only 2 to 3 feet between rows or hills.
Soil pH and fertility
For best yield and quality, the optimum soil pH range for vine crops is between 6 and 6.5, which is slightly acidic. However, most gardeners, even those with neutral or slightly basic ("alkaline") soils can successfully grow summer squash.
Nearly all garden soils benefit from the incorporation of well-rotted manure or compost before planting. Adding this organic matter improves both heavy clay soils and lighter sandy soils. Before adding any compost, manure, or fertilizer, a soil test should be performed. Without a test, it is impossible to know how much or which type of fertilizer to apply.
In midseason you should apply a side-dressing of nitrogen fertilizer such as ½ cup 46-0-0 or 1 cup 27-3-3 for each 25 feet of row. If you incorporated manure or compost, additional fertilizer applications may be reduced or eliminated, depending on how much organic matter you apply. Do not use "Weed and Feed" type fertilizers on vegetables. They contain weed killers that will kill vegetable plants.
Continuous use of high phosphorus fertilizer such as 10-10-10 or 15-30-15, or high rates of manure or manure compost results in phosphorus buildup in the soil. Although phosphate fertilizer applied to soil is bound tightly and resistant to movement in the soil, some runoff may occur. It can then become a major pollution concern in our lakes, rivers and streams. High levels of phosphorus support over-production of algae, which causes significant reduction in water quality (see Preventing Pollution Problems from Lawn and Garden Fertilizers). If your soil tests high in phosphorus, use a low phosphorus (such as 32-3-10, 27-3-3, or 25-3-12) or no phosphorus (such as 30-0-10 or 24-0-15) fertilizer.
Long taproots and branching surface roots enable squash plants to access soil moisture even in dry weather, however squash plants are heavy water feeders and soil moisture should be monitored. Supplemental irrigation will enhance production. Vine crops need at least one inch of water from rainfall or irrigation each week during the growing season. Always soak the soil thoroughly when watering. There is little or no value in a light watering that only wets the surface of the soil. On most soils, watering once a week is sufficient. Sandy soils should be watered more frequently but with lower amounts applied at any one time.
Pick summer squash when they reach the size you prefer, before they are over-large, with large seeds, hard skins, and fibrous or watery flesh. Tiny “baby” fruits are valued for their tenderness and eye-appeal. If your squash planting is large, you may choose to pick some at the “baby” stage. In a smaller planting, it makes more sense to harvest medium-sized fruits. Squash blossoms are also edible, and should be harvested the day they open. Male flowers can be picked without significantly affecting the yield of squash fruits.
If very large squash fruits are left on the vine, plant yield will decline, so remove fruits that have grown too large even if you will not use them. Harvest often, but be careful not to disturb the plants, as they often send out new roots from joints in the vine. Disturbing the vine can break these roots. Do not pick fruit when the vines are wet, because of the danger of spreading diseases.
For assistance in diagnosing unknown problems visit ‘What’s wrong with my plant?’
Squash bugs feed on foliage and can harm young plants.
Squash vine borers can kill plants as they tunnel through the vines.
Striped cucumber beetles damage plants by eating leaves as well as stems and fruit. They are also a potential vector of bacterial wilt.