When to Harvest
Ripe watermelon doesn't slip from the vine when it is ripe, so it takes a little more know-how to tell when a watermelon is ready to pick.
Use this combination of indicators to check for ripeness:
Tendrils near the fruit stem have become brown and dry;
The fruit surface is rough to the touch and the fruit color is dull;
The bottom of the watermelon (where it lies on the soil) has changed from a light green to a yellowish color;
Look for 'black sap spots'. These are caused by bees extracting the juice from the melon. If there are several beads of hard black sugars, you know you've got a sweet melon.
Preventative and Natural solutions to Common Pests and Problems
The use of row covers will help prevent infestations of most pests (see Row Covers, Hot Caps, & Plastic Tunnels)
Striped and Spotted Cucumber Beetles
These pests spread bacterial wilt throughout your crop when they feed; they tend to attack around the time the plants begin flowering. Note: Male flowers will appear first, at leaf joints on the main stem and female flowers will appear. About one week later, female flowers will develop off of secondary side-shoots.
Your best defense against these pests is to cover your plants with row covers until flowering. The covers must be removed once the female flowers appear so that the bees can access the plants for pollination.
A severe infestation can be controlled with pyrethrins; the pyrethrins are a pair of natural organic compounds that have potent insecticidal activity.
Melon Aphids
Aphids are tiny pear-shaped insects with long antennae that suck sap from the plant, causing the leaves to become distorted and drop.
A strong spray of water can often knock the insects off your plants. Crushing several cloves of garlic into a spray bottle filled with water and spraying mixture on the plants can often deter these pests. Insecticidal soap, pyrethrins, and neem oil are also effective organic insecticides for heavy infestations.
Squash Vine Borers
Squash vine borers look like 1" long white caterpillars. They bore through the stem of the plant and may be unnoticeable to the gardener until the vine begins to wilt.
In April (in the South) and in late June or early July (in the North), keep a lookout for an orange and black wasp-like moth. This is an adult borer which lays eggs at the base of the stem during this time. The tiny orange and red eggs are laid just below the surface of the soil. Remove and crush any eggs you can find.
Once the season progresses, keep an eye for entry holes at the base of the stems. If the hole is surrounded by a yellow dropping that resembles sawdust, cut a slit in the stem and remove any larvae. An injection of Bt can also be used (Bacillus thuringiensis, or Bt, is a soil-dwelling bacterium, frequently used as an organic alternative to chemical pesticides).
Environmental Factors
Vine crops that have recently been transplanted or that have been direct-seeded outside and recently germinated have been known to suddenly wilt and die. Most often this occurs when you have a stretch of 4 or more days of rainy, or overcast weather with no protection over your plants to retain heat. If soil temperature drops down below 60F, the plant roots stop absorbing water from the soil.
When the sun does reappear, water evaporates from the leaves faster than the roots absorb water, which results in sudden wilting and death. To prevent this from happening, keep your plants protected when cool wet weather is predicted with the use of row covers or hot caps. In humid climates, watering the roots directly rather than soaking the leaves helps prevent many common foliar diseases.
Disease
Most diseases can be avoided by following these rules:
When growing watermelon choose varieties resistant to common diseases such as powdery mildew. Wait until the soil has warmed to a minimum of 70F before planting seeds or transplants and keep soil temperatures consistently warm throughout the entire plant cycle. Keep seeds and seedling beds moist but not water logged.
If starting indoors, plant seeds in 4" pots or larger to lessen root disturbance and allow more room for roots to develop properly. Transplant seedlings once they have 2-3 true leaves. Avoid over-applying Nitrogen (such as animal manure). Too much nitrogen, especially during fruit set, can encourage "hollow heart" and bland flavor. Provide good air movement around plants by keeping weeds under control and planting with proper spacing.
Powdery mildew
Powdery mildew, a fungus, can cause leaves to prematurely die, and reduce yield and fruit quality. Damage can include brown spots, tattered holes in leaves, sunken brown lesions on vines, and rotted fruit. Powdery mildew infections are usually caused by warm, humid conditions (68-81F), or the plant being damp too long from rain or overhead watering.
In warm, dry conditions, new spores are produced in leaf spots and can easily spread disease by being blown by the wind and infecting other leaves. To avoid these diseases do not grow melons in an area where any member of the squash family has been grown for 3 years or more. Reduce moisture on leaves by providing plants with proper space, controlling weeds, and using drip irrigation or a soaker hose instead of sprinkler irrigation. Apply a natural fungicide when a single spot of powdery mildew is spotted. Sulfur products can be applied to both the upper and lower surfaces of the leaves. Remove severely infected plants and any debris from the garden.
Plant disease resistant varieties whenever available. Resistance is virus specific and it is necessary to first determine which mosaic virus is common in your area. A reputable seed company in your part of the country will have this information. If not, ask your county extension.
Purchase clean seed from a reputable supplier. If saving seeds, do not collect seed from infected plants. Control weeds within and around the planting area. Especially perennial weeds that can carry the virus from one season to the next. Control aphids and cucumber beetles early on to keep populations low. If disease appears in a few plants, bury or dispose these plants to prevent further spread of the disease. Wash hands and tools after working with infected plants.
Storage
Watermelons will keep in the refrigerator for up to one week, but not only will flavor and sweetness diminish, so will nutrition levels. The best way to store a watermelon is at a room temperature of 50-60°, such as a basement. Watermelons will last up to 2-3 weeks and retain their flavor, sweetness and nutrition. Melon flesh can be frozen (think watermelon smoothie) and rinds can be pickled.
