Growing pumpkins and winter squash in Minnesota home gardens
Vincent A. Fritz, Extension Horticulturist, Southern Research and Outreach Center,
Carl J. Rosen, Extension Soil Scientist, Department of Soil, Water, and Climate
Edited by Terrance Nennich, Extension Educator, Horticulture
Pumpkin, C. pepo
Turk's cap squash, C. maxima
Pumpkins and winter squash are among the most popular vine crops in the garden. The terms pumpkin and squash can be confusing. Pumpkin pie is often made from squashes, and some large squashes are used ornamentally. Scientifically speaking these plants are all very closely related members of the cucurbit family, which also includes summer squash, zucchini, and cucumbers.
Pumpkins come from two different species Cucurbita pepo (most jack o’lantern and some pie pumpkins) and C. maxima (extremely large pumpkins grown for competition and decoration). Make sure to check variety descriptions carefully when purchasing seed. Pumpkins grown for jack-o’-lanterns are usually not eaten, as the flesh is bland and stringy, although the roasted seeds are good to eat. Pie pumpkins often have smaller, sweeter fruit. Some pumpkin varieties produce “naked” or hull-less seeds especially nice for roasting, since there is no hard shell to crack from the seeds. These seeds have lower germination rates, particularly in cool soil, so they are more difficult to grow.
Edible winter squash belong to three different species: Cucurbita pepo (acorn, delicata, and spaghetti types), C. moschata (butternut types), and C. maxima (Hubbard, kabocha, and buttercup types). Some varieties produce small squashes the right size for individual servings, while others produce enormous fruits of fifteen pounds or more, suitable for soups, pies, mashing, or freezing. Some can be stored through the winter; others should be used within a few weeks after harvest. Choose varieties that suit your tastes as well as your ability to handle and store the squash. While a giant Hubbard squash may be attractive as an autumn decoration, a small household may be unable to utilize it as food. Note the days to harvest for the varieties you are considering. Longer-season varieties may be difficult to ripen properly in parts of Minnesota.
Vine crops can be seeded directly in the garden but need warm soils (65°F at 2" soil depth) to germinate properly. In Minnesota planting is typically in late May to early June. Plant pumpkin and winter squash seeds 3/4 inch deep, 24-36 inches apart. Use the closer spacing if the variety is a bush type. Spacing between rows should be 5-6 feet apart.
Pumpkins and winter squash can be transplanted to help extend the season in the Northern parts of the state, or to help guarantee a full stand. Plant seeds into pots or cells at least two inches wide and deep. The seedlings take about four weeks from seeding to transplanting and should have 2-3 true leaves at the time of transplanting. Seedlings started indoors need to be hardened off before being planted in the garden. Transplant pumpkins and winter squash into the soil when soil temperatures reach at least 65°F. Transplants are usually planted after the first flushes of weeds are removed, helping to keep weeding later in the season to a minimum.
Pumpkins and winter squash “run” on the ground and take up a lot of space. In small gardens bush-type varieties may be used. Small fruited squash like delicate or acorn can be trained to a trellis to save space. Large fruited squash are too heavy to be trellised and should be grown on the ground.
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 pumpkins and winter squash. Pumpkins and winter squash do well in heavier soils, although more fruit belly rot may occur. The soil should be well drained yet moisture retentive.
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, apply a sidedressing of nitrogen fertilizer, such as ½ cup of 46-0-0 or 1 cup of 27-3-3 for each 25 feet of row. If you use 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.
Proper watering will enhance good production. Pumpkins and winter squash need at least 1 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. Plants that are trellised to grow vertically may require closer monitoring as they will likely require more frequent watering. 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.
Frequent, shallow cultivation with a garden hoe or hand trowel will kill weeds before they become a problem. The roots of vine crops are close to the surface of the soil, so it is important not to cultivate too deeply or too close to the plants. Cultivate just deeply enough to cut the weeds off below the surface of the soil. Continue cultivating as long as you can do so without injuring the vines, usually when the vines begin to spread between the rows. When cultivation is no longer possible, pull large weeds by hand.
Pick winter squash and pumpkins before a hard freeze. A light frost that kills the vine will usually not harm the fruit. Cut the fruit from the vine, leaving a few inches of stem attached. Be careful not to cut or bruise the fruit.
Ideally, the pumpkins and squash should be field-cured in place for a week or two in dry, sunny weather. This dries and toughens the skin for longer storage. Alternatively, if the weather has turned cold or rainy, squash can be cured indoors, in a warm (80°F), well-ventilated space.
Because squashes and pumpkins used as autumn decorations are subject to cold, wet weather, they can easily spoil. Monitor the condition of the fruits and discard when they start to show signs of rot, as they can quickly disintegrate and become a foul-smelling mess on the front step.
Winter squash should be stored in a cool but not cold place, ideally around 55°F, with good air movement. Relative humidity between 50% and 75% is best. Check squash in storage frequently and remove any that are soft or show signs of spoilage. Remember to treat them gently.
If appropriate storage is not available, squash can be cooked and mashed, then frozen. Canning of mashed or pureed squash is not recommended because of the density of squash in the jars; it’s nearly impossible for the heat of the canning process to penetrate to the center of the jar.
Poor fruit set could be due to improper pollination. Pollination may be hindered by cold rain and cloudy weather. Tasteless fruit could be due to dark, cloudy weather, or disease.
Many varieties have separate sexes in flowers. This characteristic is referred to as a monoecious flowering habit. Blossom drop of male flowers is, to some extent, normal because only the female flowers produce fruit. Female flowers can be identified by the swollen ovary at the base of the flower.
Squash bugs feed on foliage and can harm young plants. Squash vine borers can kill plants as they tunnel through the vines. Wilting vines is probably the first symptoms you will notice. Striped cucumber beetles damage plants by eating leaves as well as stems and fruit. They are also a potential vector of bacterial wilt.
Powdery mildew is a common problem on pumpkins and winter squash. This fungus can be seen as a white fuzzy growth on leaves in late summer. The fungus robs nutrients from the plant, reduces yield and in some cases can even kill the plant. Plant powdery mildew resistant or tolerant varieties and promote good air movement around plants by controlling weeds and using proper plant spacing.