Winter Squash and Pumpkin

Winter Squash and Pumpkins Cucurbitaceae, Cucurbita

Place of Origin: Latin America

Optimal temperatures for germination: 77-86℉(25-30℃)

Optimal growing temperatures: 62-68℉(17-20℃)



Scientific names: Cururbita maxima (Kabocha Squash), Cucurbita moschata (Butternut Squash), Cucurbita pepo (Pumpkin and Vegetable Marrow), Cucurbita ficifolia (Figleaf Gourd)


There are four types of squash, and according to type, the proper temperature for plant growth varies.

・Kabocha squash varieties are the most vulnerable to low temperatures and grow vigorously in a hot and humid climates. The optimal temperatures for plant growth is between 77-86°F(25-30°C). The lowest temperature for root growth is 64°F(18°C).

・Butternut squash varieties like a cold climate. The lowest temperature for root growth is 42-46°F(6-8°C).

・Pumpkins and vegetable marrows resist low temperatures and have a wide range of temperatures in which they can be grown.

・Figleaf gourds are the only type of perennial squash and are not usually grown as edible squash, though their leaves, seeds, and fruit can be eaten.

When growing squash, choose what variety to grow, according to their characteristics.


Seeding and Growing Seedlings

Seedlings should be grown at a low temperature. There are two ways of growing squash; one is growing seedlings, and the other is sowing the seedsdirectly in the ground after the danger from a late frost has past. You can easily grow squash with directseeding.The optimal temperature for germination is 77°F25°C.  When growing in greenhouses while using seed trays, the space between rows should be around 3-4” (8-10 centimeters),the space between seeds should be around 1-1½” (3-4 centimeters), and the seeds should be planted about 1” (1.5centimeters) deep. After germination, keep the temperature at60-70°F(16-22°C) to prevent plants from bolting.After cotyledons appear (7-8 days after seeding), transplantseedlings to pots of 4-6” (12-15 centimeters) in diameter. Then,keep the soil temperature at 71-77°F(22-25°C) for good rooting, and then laterlet the soil temperature go down to 60-68°F(16-20° C). Keep the temperatureat 68-77°F(20-25°C) during the daytime and keep it at 59-64°F(15-18°C) duringthe night. When the first two leaves emerge after the cotyledons, keep the greenhouse’s temperature at 48-52°F(8-11°C) and keep the soil’s temperature at 57-61°F(14-16°C). Water the seedlings in the morning to match how much the surface of the soil had dried out in the previous evening.

Growing Seedlings

Temperatures change the time when fruit is produced. If squash seedlings are grown at low temperatures under short-day time conditions, female flowers come out on the earlier jointsof stems.If you grow seedlings at high temperatures under long-day time conditions, female flowers come outon the later joints of the stems. Therefore, seedlings should be grown at low temperatures toprevent female flowers from coming out on the earlier joints, which prevents a great deal of strain on the seedlings.The results reported in the vegetable dictionary show that the first femaleflower comes out on the 10th joint when the highest temperature averages 69°F(20.5°C) and the lowest temperature averages 50°F10° C and when the length of the longest day is around 8 hours.On the other hand, the first female flower comes out on the 24th joint when the highest temperature averages 80°F(26.8°C) and the lowest temperature averages 69°F(20.7°C) and when the length of the longest day is 16 hours.


Growing Seedlings in their Final Location


Plant seedlings when the lowest soil temperature reaches 54°F(12°C) or higher. The optimal time for planting the seedlings is after 4-5 leaves have emerged after the cotyledons. (The proper time for planting seedlings change according to the crop’s variety.)In order to reduce the injury caused to plants when transplanting, lower the temperatureat night for 5 days before, and don’t overwater so thatseedlings become conditioned to the outside.


On a warm, windless day, water the seedlings fully and then plant them. The width of the ridge should be around 70-78” (180-200 centimeters) and the space between seedlings should be around 31-39” (80-100 centimeters). Plant the seedlings shallowly so that the surface of the pot is 1/2-3/4” (1-2 centimeters) higher than that of the ridge (the same measurements are used when sowing directly). The seedling should be planted in such a way that 1/3 of the root clump is above the surface of the ground.  Then gather the soil around the root clump.


Kabocha squash varieties have less stems than butterbut varieties. So, when growing butternut varieties, the ridge width should be larger and the space between seedlings should be smaller.

Pruning and Training Vines

Squash can be grown with or without pruning the vines, but when the vines are pruned, a lot of regularly shaped fruit is produced. If the vines are pruned or not, they will produce fruit at the same period. When pruning Japanese pumpkin, pinch off a main stem when 5-6 leaves come out after cotyledons and grow 3-4 side stems arising from the main stem. As a lot of vines and leaves grow, pinch off unnecessary vines soon. With Western pumpkin, according to crop type, it could grow only a main stem.


Grass Mulch


Grass mulch is useful for keeping the soil temperature from rising and prevents mud from splashing on the leaves and fruit. Apply thegrass under the space where the vines will be growing.



If a lot of honeybees don’t come to flowers during the flowering period, artificial pollination is needed to produce fruit. If the outside temperatures are warm and honeybees are coming to the flowers, artificial pollination is not needed. Bumblebees also seem to be able to pollinate squash. When the flowers bloom vary from day to day. Basically, artificial pollination should be done before 9 in the morning. (If artificial pollination is done later in the day, the rate of producing fruit decreases.)


To harvest good fruit, turn the fruit 10-20 days before harvesting, this helps promote the yellow area of the fruit to ripen. Around 40 days after flowering, check to see if the stalks that link the vines to fruit become dry and withered, and then harvest the fruit. Store fruit in a shady, well-ventilated place. When fruit is produced at the 8th-10thjoint on side stems, it is said that the fruit tastes really good.

Even if kabocha squash is not completely ripe, the fruit is tasty and can be eaten. After

it is completely ripe, the skin becomes brown. Check the color of the fruit in order to be able to save seeds. Both fully ripened butternut squash and Kabocha squash can be stored for a long time. Therefore, if the squash is going to be eaten quickly, it can be harvested earlier.  However, if the fruit is going to be stored for a long time, leave the fruit on the vines until it becomes fully ripened.

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