Do Melons Continue to Ripen Off the Vine

A quick guide to harvesting and storing melons, squash, and pumpkins

Author: Natalie Hoidal, Reviewed by: Cindy Tong & Charlie Rohwer

Melons are one of the trickiest plants to grow, because the guidelines around harvest are confusing and often contradictory. I often hear growers express frustration because their melons seem ripe but don't have much flavor, or because they spoil faster than they should. This article presents an overview of ripening, as well as harvest and storage tips for melons, watermelons, squash, and pumpkins.

Some ripening basics

Fruit and vegetables are typically assigned to two categories that define their ripening behaviors:

• Climacteric: Climacteric fruits and vegetables continue to ripen off the vine. They experience a rapid increase in respiration during ripening. They also often produce a hormone called ethylene after harvest, which can speed up ripening and senescence. If eaten at an immature stage, these fruits and vegetables taste bland, and the texture is often not as soft as it is meant to be. If left at room temperature they will become sweeter. Common examples of climacteric fruit include apples, bananas, peaches, and tomatoes.
• Non-climacteric: These fruits and vegetables do not continue to ripen off the vine. Once removed, the sugar content will not increase, and so picking at exactly the right time is important for flavor. These fruits and vegetables can still go bad if left out, but they tend to have a longer shelf life than climacteric fruits and vegetables. Examples include: grapes, strawberries, and watermelon.
  

However, sometimes the lines between climacteric and non-climacteric are a bit blurry, and this is especially true in melons. Ripeness is determined by a variety of traits including sugar content, how easily the fruit detaches, ethylene production, and firmness of the skin.

One important concept in determining ripeness is the formation of an abscission layer, sometimes referred to as "full slip". Cucurbits that are fully climacteric will develop a distinct layer of cells at the point where the vine connects to the fruit, called an abscission layer. I think of this layer as a scab of sorts - these cells allow the connection point to harden off, which results in the fruit falling off the vine with a very gentle nudge. Many online articles tell you to harvest at full slip, but there are two problems with this:

• Many cucurbits, including watermelon, honeydew melons, squash, and pumpkins, never actually develop an abscission layer.
• While this is a great recommendation for gardeners who want to eat a ripe melon directly from the garden, the melons that are able to achieve "full slip" will continue to ripen off the vine, and therefore may be over-ripe by the time farmers actually get them to their customers.
  

One final consideration is ripening hormone ethylene. Some fruits and vegetables produce ethylene as they ripen, including some melons. Even if a fruit or vegetable does not produce ethylene, it can be sensitive to it. Storing ethylene-sensitive crops alongside ethylene-producing crops can lead to faster spoiling. In some cases this is done intentionally (e.g. storing green bananas in rooms full of ethylene at grocery distribution centers so they ripen quickly). But, if your goal is to store these crops as long as possible for fall and winter sales, it's best to keep ethylene-producing crops separate from sensitive crops.

So with that background, let's look a bit deeper into each of the major cucurbit groups (excluding cucumbers).

Watermelons

Watermelons all belong to the same species, Citrullus lanatus. They were likely domesticated around Libya and Egypt (Chomicki et al., 2019). All watermelons are non-climacteric, meaning they should be left on the vine until they are fully ripe. Signs of ripeness include:

• The spot where the fruit touches the ground becomes more prominent and changes color (typically yellow).
• The tendril closest to the fruit becomes brown and dries up.
• Ripe melons have a hollow, dull sound.
• The sheen of the rind tends to change slightly with maturity, but this is variety dependent.
  

Watermelons do not produce ethylene, but they are sensitive to it and should not be stored with ethylene producing crops like tomatoes, bananas, apples, or cantaloupe in order to extend the shelf life (Shrefler et al., 2017). They should be stored at very high relative humidity (95%) at 50-60 degrees fahrenheit. With proper storage, they can last 2-3 weeks.

Tendril is beginning to dry up. This watermelon will be ready soon. Photo: Lindsey Miller

The ground spot will become more pronounced as watermelons ripen. Photo: Lindsey Miller

Melons

"Melon" is a vague word in English; many other languages have distinctly different terms for the fruit belonging to the species Cucumis melo subspecies melo, and for watermelon, bitter melon, and other related, but distinct cucurbits. Melon in this article refers to Cucumis melo subspecies melo, which contains hundreds of distinct cultivars including cantaloupe and honeydew. Melons originated in both in Asia and Africa, with the oldest known melons dating back to China in 3000 BC, and lower Egypt in 3700 BC (Chomicki et al., 2019).

Melons in the species Cucumis melo subspecies melo are very diverse, both in appearance and ripening behavior. Image: Monforte et al., 2014.

There are both climacteric and non-climacteric varieties within this species. Leida et al. (2015) conducted a thorough genetic study of ripening characteristics of 175 melons from 50 countries, and defined some general groupings. While this is not a comprehensive summary, these are my main takeaways for farmers:

• Cantaloupe: Cucumis melo spp. melo var cantalupensis and Cucumis melo spp. melo var reticulatus are the two primary varieties of cantaloupe-style melons. These melons often have netted rinds, but not always, and they vary from lobed to smooth. They all have a relatively high sugar content. They have been consistently bred over time to be fully climacteric. They form a full abscission layer, and the fruit detach readily from the vine when they are ripe. While full slip is the point at which they are fully mature, they will continue to ripen off the vine if harvested just before full slip, and can be harvested early to ensure they are still fresh when they reach customers.

• To make things more complicated, breeders have begun to introduce "no-slip" traits into commercial cantaloupe varieties. These are newer traits introduced mostly into varieties destined for shipping, so they are unlikely to be grown by Minnesota farmers at this point.

• The indorus (honeydew) group: Honeydew are the most well-known type of melon in the subspecies Cucumis melo spp. melo var indorus. This group also includes common melons like Piel de Sapo. These melons have an even higher sugar content than cantaloupe melons, and they are, for the most part, non-climacteric. They will not form an abscission layer (and thus "full slip" is not an indicator of ripeness). Since they do not ripen off the vine, they are less likely to go from under ripe to overripe in a matter of days, but the storage life is still only 2-3 weeks in cold storage. Indications of ripeness vary across varieties, but common indications include rind color changes, and the presence of a sweet smell.
  

• To make things complicated again, many internet sources say that honeydew melons are climacteric. This is likely based on a paper by Passam & Bird (1978) in which the authors studied a non-climacteric honeydew melon (read below in the "harder to define melons" section), and made the broad assumption that all honeydew melons are climacteric. This newer research clarifies that while there are some climacteric honeydew melons, the majority are not.
  

• Makuwa, Chinensis, and Conomon melons and other melons from East Asia (China, Japan, Korea, and the Philippines): This group is a bit less defined than the cantaloupe and honeydew groups, because there is more genetic variation between varieties. They tend to range from non-climacteric to "weakly climacteric". Generally farmers could apply the same principles listed for the honeydew group, but make sure to read the variety descriptions in your seed catalog, and check on these melons more often in storage.
  

• The harder to define melons: Melons from Northern Africa and Western and Central Asia, in addition to some Spanish landrace melons (many of which could be classified as honeydew melons), are extremely diverse and do not fall into neat ripening categories. Instead, read the variety descriptions, and consider planting smaller test plots of these melons to observe how they behave.
  

When reading variety descriptions, check for terms like "harvest at full slip", "cut from vigorous vines when the skin begins to yellow", and "cut from vines when the skin becomes soft". Keep in mind that "harvest at full slip" may be a better recommendation for gardeners than it is for farmers, and think of it instead as a hint that it's a climacteric melon that will continue to ripen.

While gardeners should wait until cantaloupe slip easily from the vine, farmers might be better off harvesting a bit before this stage since the melons will keep ripening. Most other melons will never reach the full slip phase. Image: Natalie Hoidal

All types of melons should be stored at 95% relative humidity. Cantaloupe should be stored at 40 degrees fahrenheit, and honeydew can be stored in slightly warmer conditions, between 45-50 degrees fahrenheit. Cantaloupe produce ethylene and are sensitive to it. Honeydew melons and other non-climacteric melons do not produce ethylene and are not particularly sensitive to it if they are stored near ethylene producing fruit (Suslow et al., 1997).

Winter squash & pumpkins

Winter squash and pumpkins fall into three main species groups: Cucurbita maxima (e.g. kabocha, hubbard, arikara squash, some pumpkins)), Cucurbita moschata (e.g. butternut and some other winter squash), and Cucurbita pepo (e.g. pumpkins, zucchini, acorn, delicata, pattypan, and summer squash). Pumpkins and squash originated across Mesoamerica, with specific types originating across the continents in places including Argentina, Brazil, Guatemala, Mexico, Puerto Rico, and the Southern United States (Chomicki et al., 2019)

All types of pumpkins and squash are non-climacteric, and so they should be allowed to ripen to full maturity on the vine. Most are not ethylene sensitive, so they can be stored alongside ethylene producing crops (with the exception of summer squash). However, ethylene may cause the stems to break off of immature fruit, which can make the fruit more susceptible to secondary infections, and green-colored squashes may lose some of their green color in the presence of ethylene (Cantwell & Suslow, 2014).

As pumpkins and squash ripen, the rind will become increasingly firm, and they should not dent when you press a fingernail into the skin. The vines usually also begin to decline when squash are ready, and the stem should be firm.

Winter squash and pumpkins should be stored at 50% relative humidity, and around 50-55 degrees fahrenheit. They can last 1-6 months in storage, depending on the variety.

Sources

Cantwell, M., Suslow, T. 2014. Pumpkin: Recommendations for Maintaining Postharvest Quality. http://postharvest.ucdavis.edu/Commodity_Resources/Fact_Sheets/Datastores/Vegetables_English/?uid=28&ds=799

Chomicki, G., Schaefer, H. and Renner, S.S. (2020), Origin and domestication of Cucurbitaceae crops: insights from phylogenies, genomics and archaeology. New Phytol, 226: 1240-1255. https://doi.org/10.1111/nph.16015

Leida, C., Moser, C., Esteras, C. et al. Variability of candidate genes, genetic structure and association with sugar accumulation and climacteric behavior in a broad germplasm collection of melon (Cucumis melo L.). BMC Genet 16, 28 (2015). https://doi.org/10.1186/s12863-015-0183-2

Passam, H.C., Bird, M.C. 1978. The respiratory activity of honeydew melons during the climacteric. Journal of Experimental Botany, 29(109): 325-333.

Shrefler et al., 2017: https://extension.okstate.edu/fact-sheets/watermelon-production-2.html

Suslow, T., Cantwell, M., Mitchell, J. 1997. Honeydew: Recommendations for Maintaining Postharvest Quality. http://postharvest.ucdavis.edu/Commodity_Resources/Fact_Sheets/Datastores/Fruit_English/?uid=27&ds=798

Suslow, T., Cantwell, M., Mitchell, J. 1997. Cantaloupe Recommendations for Maintaining Postharvest Quality. http://postharvest.ucdavis.edu/Commodity_Resources/Fact_Sheets/Datastores/Fruit_English/?uid=27&ds=798

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