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CITRULLUS lanatus 'Ultra Cool'

Image of Citrullus lanatus 'Ultra Cool'



Botanical Name

CITRULLUS lanatus 'Ultra Cool'

Plant Common Name

Ultra Cool Watermelon, Watermelon

General Description

A self-sterile F1 hybrid, 'Ultra Cool' produces medium-sized, rounded, seedless watermelons with dark-striped rinds and sweet, reddish-pink flesh. Plants grow from seed to harvest in 90 to 100 days. This triploid cultivar (three sets of chromosomes) requires a diploid companion (such as 'Sugar Baby') for pollination and fruit set.

Watermelon is a tender annual tropical vine that needs a long, very warm growing season to produce its famous fruits. Yellow, bee-pollinated flowers appear among the attractive, deeply lobed, gray-green leaves throughout the growing season. Some watermelons flowers are male and others female. Female blooms have a bulbous ovary at the base, which will eventually become the fruit, and the males only have pollen-laden anthers.

Watermelons do not become sweeter after being harvested, so they must be picked when ripe. It is tricky to know when to harvest—especially considering the fruits take a while to mature and patience can wane. The best means is to monitor the tendril closest to the developing fruit. Once the tendril starts to turn brown, the fruit is ready. Another method is to keep an eye on stem health. When the stem is green and firm, the melon is still ripening; a soft withering green stem is an indication of ripeness, and a dry or unattached stem can mean over-ripeness. Finally, check the underside of the melon and give it a light knock. If the underside of the melon has turned from white to pale yellow and a hollow sound emanates from the fruit, it is probably ripe. When harvesting, cut the melon from its stem. Tearing the stem can lead to vine rot.

Full sun and fertile, crumbly-textured sandy loams are perfect for watermelon culture. Watermelon plants appreciate sharp drainage, so it is best to sow seeds in low mounds of soil (called "hills"). After the threat of frost has passed, plant as many as three seeds per hill and keep the soil evenly moist but not wet (wet soil can induce seed rot). Alternatively, plant seed in small pots (using a well-drained potting mix) and set out seedlings after the last frost date. Provide ample room for the plants to sprawl across the ground. The sheer weight and size of the melons precludes training the vines on trellises.


  • AHS Heat Zone

    12 - 4

  • Sunset Zone

    H1, H2, 1a, 1b, 2a, 2b, 3a, 3b, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24

  • Plant Type


  • Sun Exposure

    Full Sun

  • Height

    1'-2' / 0.3m - 0.6m

  • Width

    6'-8' / 1.8m - 2.4m

  • Bloom Time


  • Native To

    Africa, Southern Africa

Growing Conditions

  • Soil pH


  • Soil Drainage

    Well Drained

  • Soil type


  • Growth Rate

    Very Fast

  • Water Requirements

    Average Water

  • Habit


  • Seasonal Interest

    Summer, Fall

Ornamental Features

  • Flower Interest


  • Flower Color


  • Fruit Color

    Light Green, Dark Green

  • Fruit Color Modifier

    Bicolor, Striped/Striated

  • Foliage Color (Spring)

    Gray Green

  • Foliage Color (Summer)

    Gray Green

  • Foliage Color (Fall)

    Gray Green

  • Fragrant Flowers


  • Fragrant Fruit


  • Fragrant Foliage


  • Bark or Stem Fragrant


  • Flower Petal Number


  • Repeat Bloomer


  • Showy Fruit


  • Edible Fruit


  • Showy Foliage


  • Foliage Texture


  • Foliage Sheen


  • Evergreen


  • Showy Bark


Special Characteristics

  • Usage

    Edible, Fruit / Fruit Tree, Herb / Vegetable, Tropical

  • Sharp or Has Thorns


  • Invasive


  • Self-Sowing