Choosing a Ripe Watermelon (for field harvest)

Watermelon Crop Information

  • by Todd C. Wehner
  • Department of Horticultural Science
  • North Carolina State University
  • Raleigh, NC 27695-7609

It is difficult to choose a watermelon fruit in the field and know that it has reached its optimum harvest stage. The usual criteria for choosing a ripe fruit are as follows:

  • begin checking after the flowers have been pollinated (it often takes 2 to 5 weeks from pollination to harvest, depending on temperature)
  • count the number of days from planting to harvest (it may take 60-90 days to reach harvest, depending on cultivar)
  • check the fruit rind to see if it has changed from shiny to dull
  • look at the ground spot (where the fruit contacts the soil or mulch) to see if it has become light colored
  • check the tendril where the fruit attaches to the stem to see if it has changed from green to brown
  • thump the fruit to determine whether the sound has gone from bright to dull

After checking the above criteria, some of the fruit in the field should be cut to determine whether the particular cultivar being grown is maturing according to the indicators listed. Some cultivars do not have the fruit ripeness expected by the looking at those criteria. For example, mini watermelons (fruit weight <9 lb) often do not express the traits listed above, even when the fruit are ripe.

Some cultivars have a ground spot that changes color from cream to yellow. For example, ‘Black Diamond Yellow Belly’ has a yellow ground spot.

Watermelon growers who will be shipping the fruit a long distance to market may want to harvest the fruit early to provide a longer shelf life.

In order to be marketable, the watermelon fruit should be sweet. A refractometer reading of 10 °brix (10% soluble solids) should be considered the minimum acceptable. Soluble solids consist mostly of sugars in watermelon fruit, so the refractometer reading will be a good indicator of sweetness. However, the fructose and sucrose content of the soluble solids fraction does depend on cultivar. Since fructose tastes sweeter than sucrose, it is possible to have one fruit that tastes sweeter than another having the same refractometer reading (°brix).