When to Plant
A long growing season (at least 3 months of warm weather) is needed to mature and develop sweet watermelons. In short summer areas, a warm indoor start can count towards the long-season requirements. If you live in the North, starting your watermelon seeds indoors 2-4 weeks before your last frost date will allow you to take advantage of any warm, sunny days outdoors and still be able to bring them in at night or on cooler days. Planting too early will cause the plants to develop tendrils or more than four leaves which may cause your plants to have difficulty setting roots once they are transplanted to the garden. In the warmer southern climates, watermelons can be direct-seeded in the garden once soil temperatures have reached a steady 75-80F.
Where to Plant
The key to growing a successful watermelon crop is to provide as many sunny, warm days as possible for the growing plants and protect them from cold temperatures. The more exposure warmth the vines receive, the more fruit your plants will produce in the summer.
Watermelons require full sun and plenty of heat. Give your watermelons the sunniest spot available with good air circulation. This will help the plants dry out quickly after a rain to prevent diseases.
Preparing the Soil
Watermelons prefer well-drained, sandy loam soils with a pH level between 6.0 and 6.5. Soils with a pH less than 6.0 will produce plants with yellowed foliage and fewer perfect flowers. Good drainage is critical for preventing disease in your melon plants. If your soil doesn't drain water well, mix a generous amount of organic matter into the soil before planting. We recommend mounding your soil into hills. Raised hills improve drainage and helps keep the soil temperatures up. If you water with soaker hoses, mounding up a row (rather than hills) will make watering easier.
Mix in a shovel full of well-aged manure where you plan each hill. This is especially important if a spring crop has already been planted in this area and taken nutrients from the soil. Mound your manure amended soil into hills about 12" tall and 2'-3' wide. Space each hill 4'-6' apart, depending on how much space your variety needs. Adding 2-3" of aged manure or compost before planting your melons will give the plants a nutrient boost and improve your soil's structure.
As much as I'm not a fan of plastic, it's almost a necessity when trying to grow watermelons in a shorter growing season. This is something you will need to decide for yourself. Watermelons build up sugar and develop sweetness in their final days of growth so providing the necessary warmth is critical for a growing a good watermelon crop.
Covering your planting area with plastic a couple weeks before transplanting will warm the soil more quickly. Depending on the weather, this can allow you to transplant your seedlings outdoors up to 7 to 10 days earlier than without the use of plastic.
If plastic is left throughout the growing season, it will reduce ground rot in your fruit (fruits rotting from sitting on soil). Clear plastic mulch will heat the soil up 4-5 degrees warmer but black plastic will prevent weed growth.
Ventilation holes must be made in your plastic mulch to allow oxygen into the soil. If your plastic doesn't have pre-made ventilation holes and it is still on the roll, you can use a ¾" drill and drill your own holes through the layers at an 8"x8" spacing.
Choosing the Right Seeds
When choosing a watermelon variety, consider how much space the plant requires to spread out. If you have a limited growing area, choose a bush type that is more compact. Some heirloom or more vigorous varieties can take up 100 square feet per plant and only produce 2-3 fruits.
Seeds and Germination
Watermelon seeds germinate optimally between 70 and 90 degrees F. At these temperatures, seeds will germinate within 5-10 days. At temperatures below 60F, germination will not occur.
Getting Started Indoors
Watermelons don't survive frost or maintain proper growth at temperatures below 60F. In areas with short summers, seeds must be started inside and later transplanted outdoors; be extremely careful not to disturb the roots. Watermelon plants do not thrive well if their roots are disturbed, even when young seedlings.
Plant your seeds in loose potting soil (not seed germinating mix as it contains no nutrients for the developing seedling).
Plant 2 seeds ½" deep per 4" individual pot (I plant 2 seeds in case one seed does not germinate, and as the seeds get older I might add 1 or 2 more just to be sure I get one good plant). Watermelon seeds remain useable for 5 years after your initial purchase.
Keep planted seeds in a warm, sunny area (such as a south-facing window) where soil temperatures maintain between 70F-95F. Use bottom heat if necessary to maintain around 75F soil temperature. Once your seedlings are 2" tall, thin each pot to the one strongest plant by cutting the other seedlings off at soil level.
Transplanting to Outdoors
Starting your seeds indoors can extend your melon growing season 2 to 4 weeks but melons are particularly sensitive to transplanting. If the roots are disturbed, your plants growth will be stunted.
Vines may not set fruit if they're chilled when seedlings so be sure the soil temperatures remain steady at all times and the garden soil has warmed to 70-80F before transplanting outside. Planting in cooler soil will also increase the chance of soil-borne root diseases developing.
To prevent disturbing the roots, 4" use peat pots or other bio-degradable planting pots instead of plastic seedling pots. Peat pots will decompose quickly and allow you to plant the pot directly in the garden bed when you transplant the seedlings.
Carefully tear or cut out the bottom of the peat pot before planting in the ground to allow the roots to spread more quickly. Be very careful not to cut the roots. If the roots are too dense (root bound) at the bottom, don't cut the bottom out as you may damage the roots.
Transplants should have 2-3 mature leaves and a well-developed root system when they are moved into the garden. Planting seedlings that have developed 4 or more leaves may have difficulty setting roots once transplanted to the garden.
If you covered your soil with plastic to help bring up the soil temperature, you can remove the plastic before planting or leave the plastic down to continue locking in the heat. Cut slits in the plastic every 4 to 6 feet and plant seedlings through the slits.
If you choose to leave the plastic down, be sure to anchor it well. Plastic that is not anchored well can blow off during a windy day, breaking your plants in the process. If growing watermelon in rows, space seedlings 18 to 24 inches apart within the row. Space rows 5 to 6 feet apart. In shorter season areas, only plant one plant per hill so it will not compete for nutrients with other vines. In warmer climates, 2-3 plants can be planted per hill.
Crop Covers
To protect your young plants from wind damage, which can retard growth and reduce the amount of fruit the vine will set, place hot caps, plastic tunnels, or a row cover over the plants the first 2-3 weeks until the plants have a chance to better establish themselves.
The use of one of these coverings will also protect your plants from cooler air temperatures and early insect pests. Remove the covering within 2-3 weeks to avoid high temperature plant injury and to allow bees and other pollinators easy access to the flowers.
