Watermelon: Direct Sow Vegetable, May be Started Indoors
How to Sow and Plant
Sowing Seed Indoors:
- Direct sowing is recommended, but to get a head start you can start watermelon indoors 3-4 weeks before the last frost in individual biodegradable pots indoors. Sow 2-3 seeds per pot.
- Sow seeds 1 inch deep in seed-starting formula
- Keep the soil moist at 70 degrees F
- Seedlings emerge in 7-10 days
- As soon as seedlings emerge, provide plenty of light on a sunny windowsill or grow seedlings 3-4 inches beneath fluorescent plant lights turned on 16 hours per day, off for 8 hours at night. Raise the lights as the plants grow taller. Incandescent bulbs will not work for this process because they will get too hot. Most plants require a dark period to grow, do not leave lights on for 24 hours.
- Seedlings do not need much fertilizer, feed when they are 3-4 weeks old using a starter solution (half strength of a complete indoor houseplant food) according to manufacturer’s directions.
- Thin to one plant per pot.
- Before planting in the garden, seedling plants need to be “hardened off”. Accustom young plants to outdoor conditions by moving them to a sheltered place outside for a week. Be sure to protect them from wind and hot sun at first. If frost threatens at night, cover or bring containers indoors, then take them out again in the morning. This hardening off process toughens the plant’s cell structure and reduces transplant shock and scalding.
Sowing Directly in the Garden
- Sow in fertile, warm soil after danger of frost has passed.
- Sow seeds 3 inches apart in groups of 4-6 in raised hills. Cover with 1 inch of fine soil and firm lightly.
- Space groups 5-7 feet apart each way.
- Keep evenly moist.
- Seedlings emerge in 7-10 days.
- Thin to 2 or 3 strongest seedlings in each group when they are 1-2 inches high.
- PLEASE NOTE: Seedless varieties do not produce pollen and therefore cannot pollinate themselves. A pollinator (seeded variety) is included to insure a good fruit harvest both seedless and the pollinator seed must be planted together.
How to Grow
- Keep weeds under control during the growing season. Weeds compete with plants for water, space and nutrients, so control them by either cultivating often or use a mulch to prevent their seeds from germinating.
- Watermelon plans have a shallow root system, mulches help retain soil moisture and maintain even soil temperatures.
- Keep plants well-watered during the growing season, especially during dry spells. Plants need about 1-2 inches of rain per week during the growing season. Use a rain gauge to check to see if you need to add water. It's best to water with a drip or trickle system that delivers water at low pressure at the soil level. If you water with overhead sprinklers, water early in the day so the foliage has time to dry off before evening, to minimize disease problems. Keep the soil moist but not saturated.
- Do not move the vines, they are easily injured.
- Monitor for pests and diseases. Check with your local Cooperative Extension Service for pest controls recommended for your area.
Harvest & Preserving
- Watermelons are ripe when the tendrils next to the fruit turn brown.
- The spot where the fruit rested on the ground will turn to a creamy yellow.
- When you rap on the watermelon it will have a dull or hollow thud rather than a sharp sound.
- To avoid damage, cut the watermelon off the vine with sharp shears.
- Watermelon may be stored for one to two weeks in the refrigerator. For cut watermelon, wrap the cut side in plastic wrap and store in the fridge for 3-4 days.
Common Disease Problems
Alternaria Leaf Spot: Small, round reddish brown spots, usually with a yellow halo, form on the upper surface of the leaves. Severely infected leaves turn brown, curl upward, wither and die. Fruit are not usually infected but can suffer from sunscald due to leaf loss. This disease is worse in warm, wet or very humid weather. Burpee Recommends: Avoid getting water on the foliage. Remove infected plant parts and do not work around wet plants. Provide plenty of air circulation. Contact your Cooperative Extension Service for fungicide recommendations.
Anthracnose: This is a fungus disease that attacks the fruit as it is ripening. Irregular brown spots develop on the leaves. Infected fruit develop sunken black spots that may have white mycelia during wet weather. The spots enlarge and turn black; the fruit rots. Extended periods of heat and humidity facilitate anthracnose growth. The fungus overwinters in diseased plant debris. Burpee Recommends: Provide sufficient space between plants for good air circulation, avoid overhead watering which can spread the fungus spores, keep a clean garden, remove and discard all diseased plant material and rotate crops.
Bacterial Wilt: Leaves turn brown, stems wilt and shrivel, the infected plants die. Burpee Recommends: Remove and destroy plants showing signs of the disease. Control cucumber beetles, which spread the disease. Contact your Cooperative Extension Service for fungicide recommendations.
Powdery Mildew: This fungus disease occurs on the top of the leaves in humid weather conditions. The leaves appear to have a whitish or greyish surface and may curl. Burpee Recommends: Avoid powdery mildew by providing good air circulation for the plants by good spacing and pruning. Contact your Cooperative Extension Service for fungicide recommendations.
Seed Rot and Damping Off: This is a fungus disease that affects seeds and seedlings. Infected seeds will not spout. Infected seedlings can have brown thin stems and the plants will quickly die. Burpee Recommends: Do not sow seeds until the soil has warmed to 65 degrees. Plant seeds in a raised hill that will warm up earlier. Keep beds moist but not water logged.
Common Pest and Cultural Problems
Aphids: Greenish, red, black or peach colored sucking insects can spread disease as they feed on the undersides of leaves. They leave a sticky residue on foliage that attracts ants. Burpee Recommends: Introduce or attract natural predators into your garden such as lady beetles and wasps who feed on aphids. You can also wash them off with a strong spray, or use an insecticidal soap.
Cucumber Beetles: Beetles may be spotted, striped or banded and can be very harmful. Beetles are usually ¼ to ½ inch in size. Beetles start feeding as soon as they hatch and can kill or slow the growth of the plants. Beetle larva can also bore through the roots of the plants. Beetles can also transmit diseases from plant to plant. Burpee Recommends: Knock off adults into a jar of soapy water and destroy them. Spade the soil to destroy dormant beetles before you plant. Use a row cover to prevent adults from feeding on young plants. Consult your Cooperative Extension Service for insecticide recommendations.
Fruit Splitting: This indicates that when the fruit was forming it did not get an even supply of moisture from the roots. A sudden rush of water from sudden and heavy summer rains through the stem can pop the skin of a ripening fruit like an overfilled water balloon. The condition is particularly pronounced after a drought when a summer storm delivers a great amount of water to the tissues in the fruit. The skin cannot expand fast enough and splitting appears. Burpee Recommends: Take care with your watering: instead of a quick sprinkle every day, water deeply once or twice a week (depending on rainfall) so the moisture soaks deeply into the soil where roots can take it up as needed. Soaker hoses can help. Stick your finger into the soil every day to check that it is evenly moist a couple of inches below the surface.
Leafminers: These insects bore just under the leaf surface causing irregular serpentine lines. The larvae are yellow cylindrical maggots and the adults are small black and yellow flies. They do not usually kill plants, but disfigure the foliage. Burpee Recommends: Remove affected foliage.
Spider mites: These tiny spider-like pests are about the size of a grain of pepper. They may be red, black, brown or yellow. They suck on the plant juices removing chlorophyll and injecting toxins which cause white dots on the foliage. There is often webbing visible on the plant. They cause the foliage to turn yellow and become dry and stippled. They multiply quickly and thrive in dry conditions. Burpee Recommends: Spider mites may be controlled with a forceful spray every other day. Try hot pepper wax or insecticidal soap. Check with your Cooperative Extension Service for miticide recommendations.
Watermelon FAQs
Why does my fruit have a bitter or poor flavor? Bitter flavor can be caused by a number of factors including: hot and dry weather conditions, overwatering, or poor soil conditions. Melons have shallow roots; make sure the soil is moist but never waterlogged. Test your soil for nutrient deficiencies.
Why do I have flowers but no fruit? Watermelons are “Dioecious”, meaning that they have both male and female flowers on the same plant. Male flowers usually open first and female flowers have a baby fruit behind the flower. Both types of flowers must be present to have fruit set. Flowers may abort because of heat or cold or drought stress OR flowers abort because of a lack of pollination OR the plant is not ready to set fruit and the plant is self-aborting flowers to conserve and direct energy towards more growth.
Why are there seeds in my seedless watermelon? This is the seeded pollinator watermelon, not the seedless watermelon. The seeded watermelon is included to produce the pollen needed for the seedless to produce fruit. It is important to mark the pollinators so you know which melons will produce seed.
How can I tell which seeds are the pollinator plants? The pollinator seeds are larger and darker than the seedless variety.
Why is the center hollow? This is known as Hollow Heart and can be caused by cold snaps, extreme heat, or a dry spell followed by a lot of rain.