Growing summer squash and zucchini in home gardens

• Have your soil tested. 
  • Apply phosphorus (P) and potassium (K) according to soil test recommendations.  Many Minnesota soils have enough phosphorus.
  • Unless your soil test report specifically recommends additional phosphorus, use a low- or no-phosphorus fertilizer.
• For good yield and quality, the best soil pH range for vine crops is between 6.0 and 6.5, which is slightly acidic.
• Most gardeners, even those with neutral or slightly basic ("alkaline") soils, can successfully grow squash.
• The soil should be moisture retentive yet well drained.
• Forming raised beds will ensure good drainage, which these crops need. 
•  You can improve your soil by adding well-rotted manure or compost in spring or fall. Do not use fresh manure as it may contain harmful bacteria and may increase weed problems. 
  • If you use manure or compost, you may not need additional fertilizer applications, depending on how much organic matter you use.
• Side dress with fertilizer when the plants begin to spread out their vines, using one-half cup of 46-0-0, or one cup of 27-3-3, or 3-½ cups 10-3-1 for each 100 feet of row.
• Do not use “Weed and Feed” type fertilizers on vegetables. They contain weed killers that will kill vegetable plants.

Direct seeding is preferred for starting squash.

Squash seeds will not germinate in cold soil. Plants started indoors and set out into cold soil will also not grow very well. Squash plants started indoors also fail to grow in the garden because of damage during transplanting.

Use a soil thermometer and sow seeds after the last frost date, once soil temperatures are at least 70° F at the two-inch depth. In most of Minnesota, this will be sometime in late May, or early June in the north.

Some varieties of summer squash grow fruit over a longer season. Some grow many fruit within a shorter period. For a longer, more even harvest, seed only part of your squash garden at first, and then finish the planting three weeks later.

Sow seeds about a half-inch deep. For vining types that will spread out in the garden, sow their seeds two inches apart. Allow about two or three feet of space on either side of the row for the vines to spread.

After emergence, thin seedlings to stand eight to 12 inches apart. A “hill” of three or four seeds sown close together is another way to plant squash in the garden. Allow five to six feet between hills.

You can plant bush types, with very short vines, in closely spaced hills, or in closely spaced rows, with only two to three feet between rows or hills.

Earlier planting is possible with black plastic mulch, which raises soil temperature. If uncovered, the black color of the mulch will absorb heat from the sun and warm the soil faster. 

Apply black mulch after you prepare the soil in spring. 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.

Spun row covers raise air temperature around the plants and protect them from cold nights. Row covers keep away both pests and beneficial insects required to pollinate the flowers, so you must remove them from the plants once flowering begins.

• Long taproots and branching surface roots help squash plants to access soil moisture, even in dry weather.
• Squash plants are heavy water feeders, so you should make sure they have enough soil moisture. Always soak the soil thoroughly when watering.
• Vine crops need about one inch of water from rainfall or irrigation each week during the growing season.
• Water sandy soils more often, but with lower amounts applied at any one time.
• Drip hose, soaker hose or careful watering of the soil, so that the leaves stay dry, are ways to water these crops. Do not use a sprinkler or spray the plants with a hose.
• Plants that you have trellised to grow vertically may need watering more often.

• Frequent, shallow cultivation with a hoe or hand tool will kill weeds before they become a problem.
• Scratch the soil surface with a hoe just deeply enough to cut the weeds off below the surface of the soil. It is important not to cultivate too deeply or too close to the plants.
• Continue cultivating as long as you can do so without injuring the plants. This is usually when the vines begin to spread between the rows.
• When cultivation is no longer possible, pull large weeds by hand.
• If you use mulch such as straw or compost to help control weeds, do not apply it until the soil temperature is at least 75°F. These mulches can slow soil warming.

For assistance in diagnosing unknown problems, visit the University of Minnesota Extension diagnostic site “What’s wrong with my plant?”

Poor fruit set could be due to not enough pollination. Cold, rain and cloudy weather can hurt pollination. Tasteless fruit could be due to dark, cloudy weather or disease.

The first blossoms often drop from the vines, but this is not a problem, since the first flowers to appear on the vines are male. The female flowers, which open later, have a swelling at the base that forms the fruit, also known as the ovary. After bees pollinate these female flowers, the fruit develops.

• There are a few insects that sometimes feed on squash plants and fruits.  
• Squash bugs live in large groups and feed on the leaves of squash. 
• Squash vine borers can kill plants as they tunnel through the vines. This insect may cause a sudden wilting of the vines. 
• Striped cucumber beetles damage plants by eating leaves as well as flowers, stems and fruit. They are also a potential carrier of bacterial wilt.

• Many diseases can infect squash and other plants in the cucurbit family. 
• Squash seedlings are vulnerable to damping off especially when planted into cold, wet soils. 
• Powdery mildew can infect squash. It is a fungal disease that causes powdery white spots to form on leaves and vines.
  • If this disease has been a problem in the past, plant powdery mildew resistant or tolerant varieties, and promote good air movement around plants by controlling weeds and using proper plant spacing. 
• Several fungal leaf spot and fruit rot diseases can affect squash. Damage can include brown spots, tattered holes in leaves, sunken brown lesions on vines and rotted fruit.
• Virus diseases can not only harm squash plants, but also spread to other plants in the garden, including many weeds, tomatoes, lettuce, and spinach.
  • Viruses can also overwinter in perennial weeds outside the garden, and re-infect garden plants the following year. 
• Use good cultural control practices to avoid these diseases and control them if they arise.

Harvest

• Pick summer squash when they reach the size you prefer, but before they are over-large, with large seeds, hard skins and fibrous or watery flesh.
• Tiny “baby” fruits are tenderer. 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 you should harvest them the day they open. You can pick male flowers without hurting the yield of squash fruits.
• If you leave very large squash fruits 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.

Storage

• Pick and eat summer squash as it ripens.
• You can keep summer squash in the refrigerator for up to four days.
• The longer you store summer squash, the more likely the cold is to damage it.  The summer squash will develop pitted skin and water-soaked flesh.
• You can freeze, pickle and can summer squash using a pressure canner.