Seedless Triploid Watermelon Production

TRIPLOID WATERMELON (Citrullus lanatus var. lanatus)

Hybrid triploid watermelons, commonly called seedless watermelons, are generally more difficult to grow than diploid varieties. They can be grown successfully, though, with some extra care and management. The seedless trait results from crossing a normal seeded diploid parent with a tetraploid parent.

Although the fruit is considered seedless, small, edible white ovules are commonly present, and an occasional typical black seed may appear if plants are exposed to stress.

SITE SELECTION

Watermelon prefers a light, well-drained soil with a pH near 7.0. A location with a southern exposure is ideal, since it provides additional warmth. Soil moisture is especially important during the early stages of growth and again during pollination and fruit set.

Use drip irrigation to avoid excess water on the foliage, which can encourage disease. In the final week before fruit ripens, avoid overwatering, since excess moisture can lead to bland fruit.

To maintain warm soil temperatures, plastic mulch and row covers are recommended. In cooler regions, clear mulch generally warms the soil better than black mulch. Plastic mulch also helps suppress weeds around drip lines.

SEED STARTING

Triploid seeds germinate weakly and are more sensitive to cool or wet conditions than standard watermelon seed. Failure often occurs when the seed-starting mix is too cold or too wet.

Delay sowing the seedless watermelon and pollinizer until about 3 weeks before outdoor soil temperatures are expected to reach at least 65°F / 18°C and danger of frost has passed.

Sow the triploid seed and the supplied diploid pollinizer at the same time, in separate trays. Fill 2- to 2½-inch cell trays with soil-less mix and sow 1–2 seeds per cell, covering them ½ inch deep with the pointed end up or at a 45-degree angle to help the seed coat shed during emergence.

Cover trays with poly sheeting to prevent drying, but keep them out of direct sunlight so they do not overheat. If the mix dries before seedlings emerge, mist with warm water while keeping the surface on the dry side.

SEEDLING DEVELOPMENT

Seedlings usually emerge in 2–3 days. As soon as emergence begins, move trays into full sun in the greenhouse. Maintain the greenhouse at 85°F / 29°C until full emergence; 70°F / 21°C is a practical night minimum.

Thin to one plant per cell by clipping extras at the soil line rather than pulling them. Once fully emerged, reduce greenhouse temperatures to about 75°F / 24°C during the day and 65°F / 18°C at night. Irrigate with water preheated to roughly 70–75°F / 21–24°C.

ROW COVERS

Row covers can help hold the warm conditions needed for a successful crop, improve earliness, and often increase yields, especially in northern regions. Remove row covers when plants begin producing female flowers, recognized by the tiny fruit at the base of the bloom.

TRANSPLANTING

Seedlings are usually ready for the field about 3 weeks after sowing. Plants should have 2–3 true leaves and a well- developed root system that holds the soil plug together when removed from the cell.

Handle young plants carefully. Smaller seedlings may be too tender, while oversized plants—especially those with 4 or more true leaves—can suffer more transplant shock, delaying growth and reducing fruit size and total yield.

Remove seedlings gently to avoid tearing roots. Transplant only after frost danger has passed and midday soil temperatures are consistently at least 65°F / 18°C at a 4-inch depth. Cooler soils will delay growth.

Space plants 18–24 inches apart in the row with rows 6 feet apart. At transplanting, water in with a high-phosphorus starter fertilizer.

INTERPLANTING A POLLINIZER VARIETY

A diploid variety must be interplanted with the triploid variety to supply pollen for fruit production. Ace is a good pollinizer, but the broader point is that a seeded diploid pollinizer must be present. A ratio of at least 1 pollinizer for every 3 triploid plants is recommended.

Unlike regular diploids, some pollinizer varieties used with triploids take up little additional field space when set within existing rows. For many larger growers, conventional spacing may be more efficient so long as pollen is supplied throughout the bloom period.

Common pollinizer layout options

Method	How it is arranged	Best Use
Interplanted	Place one pollinizer for about every three triploid plants within the row pattern.	Smaller plantings; close pollen coverage
Row method	Use dedicated pollinizer rows so pollen is available across the block during female bloom.	Larger fields; simpler field management

POLLINATION

Pollination is essential for fruit growth. Honeybees are effective pollinators, and it is critical to have adequate bee activity when the crop is blooming. The number of hives needed varies with conditions, but a common rule of thumb is about 3 hives for every 2 acres.

FERTILIZATION

Nutrient requirements for triploids are broadly similar to those of diploids, but nitrogen management should be watched more carefully. Avoid excess nitrogen, since it can contribute to hollow heart. Triploids are considered more susceptible to hollow heart than diploids.

DISEASES

Reduce disease pressure by selecting varieties with resistance suited to your area and by keeping foliage dry through good irrigation practices and airflow.

PESTS

Protect plants from cucumber beetles with row covers applied at transplanting, or manage with appropriate controls such as pyrethrin or azadirachtin where those fit your production system.

HARVEST

Watermelon ripeness is best judged by several signs together rather than one signal alone.

• The tendril nearest the point where the fruit attaches to the vine has browned or dried.
• The ground spot—the place where the fruit rested on the soil—has turned yellow.
• The fruit gives a deeper, classic “mystery” sound when flicked, rather than a bright ping.

STORAGE

Store harvested fruit at 45°F / 7°C and about 85% relative humidity. Under those conditions, fruit generally keeps 2–3 weeks.