By Caleb Goossen
Winter squash look ready to harvest before they actually are mature. It is important to wait for maturity to maximize storage life and eating quality, whenever possible. The fruit of most squash varieties reach full size by 20 days after pollination (fruit set). Accumulation of starch and other dry matter peaks at about 30-35 days after pollination, but the fruit is not fully mature until the seeds are fully developed, which occurs about 55 days after pollination. This can vary somewhat by squash species (Cucurbita maxima, C. moschata and C. pepo) and variety, however, and a hot dry season can sometimes push squash to mature faster. Keeping track of “days after pollination” is perhaps a less common task, but it may be easier for growers that practice cucumber beetle exclusion to get “in the ballpark” if they record the date when row cover or exclusion netting was removed to first allow pollinators access to flowers.
Dr. Brent Loy, late University of New Hampshire professor emeritus and New Hampshire Agricultural Experiment Station researcher, was an expert on this subject, and stressed the importance of maintaining healthy plants until at least 50 days after fruit set because photosynthesis is essential to the development of sugars and dry matter in the fruit. A squash that is picked too early will continue to develop seeds, but it does so by depleting dry matter of the fruit, thereby reducing eating quality. However, fruit should still be harvested if squash or pumpkin plants lose their leaf coverage prematurely, whether because of powdery mildew or downy mildew or some other cause, as the fruit quality and dry matter content stop improving, and exposed fruit are left susceptible to sunburn and pest and disease risks. Foliar diseases can also greatly reduce the quality of pumpkin handles (stems), which reduces their usefulness and marketability as jack-o’-lantern pumpkins. The other important factor influencing harvest timing is the threat of chilling injury, which occurs incrementally any time the fruits are exposed to temperatures below 50 degrees Fahrenheit. Chilling injury is not as severe as frost or freezing damage, but chilling injury accumulates and the more hours that fruit are exposed to chilling temperatures the greater their potential for storage lifespan to be negatively impacted.
Curing and storage
Although fruit and seed maturity are similar across the three main species of edible winter squashes and pumpkins, curing and storage recommendations vary by type. Greater detail follows below, but in general, the Cucurbita pepo varieties require little to no storage before eating (provided they were allowed to reach maturity before harvest), while the C. maxima and C. moschata varieties may benefit from curing and almost always require at least some time in storage before they will have a desirable eating quality. The time between harvest and eating is critical, for the squash varieties that require it, to develop flavor and transform starches into sugars; however, not all varieties require it for acceptable eating quality and curing may even reduce some varieties’ storage lifespan.
For the varieties that benefit from it, curing is best performed over 5-10 days following harvest, at a relatively high temperature (80-85 F) and relative humidity (80-85%). These conditions help to harden the skin of the fruits and accelerate wound healing, as well as to kick-start the conversion of starches to sugars inside the fruit. A greenhouse or high tunnel is often an ideal location to cure squash, provided that temperatures are not allowed to get too high or too low (below 60 F), and ideally with consideration given to protecting fruit from sun damage. If squash is intended to be in storage for several months, a separate curing step is not always necessary.
The ideal storage temperatures for pumpkins and squash are between 50-60 F because that is the sweet spot where fruit does not accumulate chilling injury, but is cool enough to slow respiration — which gradually reduces fruit weight. If possible, relative humidity should be kept between 50-70% to minimize desiccation or decay organism growth. Strive to only be placing fruit into storage that is free of signs of disease, pests or unhealed wounds.
Cucurbita maxima | Kabocha, hubbard and buttercup squashes
Kabocha, hubbard and buttercup (C. maxima) varieties can be harvested before complete seed maturation, at about 40 to 45 days after fruit set, when the fruit is still bright. That’s when the rind is hardest, making the fruit less likely to be damaged in storage. Stems will typically become dry and corky by harvest. The fruit are susceptible to sunburn as the vines die back, so in cases of foliar disease, it’s best to get them harvested and out of direct sun to prevent the rind from turning brown or, with extreme sunburn, white. Kabocha squash have a high dry matter content and small seed cavity, making seed maturation off the vine less of a concern, however, once harvested they should be cured and then stored at room temperature for a minimum of 10 to 20 days to allow sugars to reach acceptable levels. Most C. maxima varieties (other than mini-kabochas and red-skinned kabochas) are starchy at harvest, however, and do not reach optimal eating quality until one to two months of storage time have passed. C. maxima varieties can often store for four to six months after harvest.
C. pepo | Acorn squash, most pie pumpkins, delicata squash and spaghetti squash
Varieties of this species can typically be eaten at harvest, if allowed to fully mature, and may even have reduced storage lifespan. With the exception of spaghetti squash, these varieties will often store for two to three months. Spaghetti squash often store poorly.
Pie pumpkins should be harvested after their skin fully turns orange, when possible. A mostly orange fruit will continue to color up off the plant however. Acorn squash are misleading because they reach full size and develop a dark green-to-black mature color about two weeks after fruit set — 40 to 50 days before they should be harvested. Loy recommended that a better way to judge maturity is to look at the rind where it touches the ground. Immature squash have a light green or light yellow ground color, whereas mature squash have a dark orange ground spot. Immature acorn squash have low sugar levels and although they will develop sweetness after harvest, they do so by depleting the dry starchy matter to convert it to sugars. This means storage life is shortened and eating quality declines.
C. moschata | Butternut squash, some pie pumpkins
Butternut squash (C. moschata) are easier to judge by sight because they don’t acquire their characteristic tan color until late in development, 35 days or more after fruit set. If the weather stays frost-free, they should be allowed to remain on the plants until 55 days after fruit set (two to three weeks after turning tan). Butternut squash should be stored for 60 days at 56-60 F, with relative humidity between 50-70% to allow starch conversion into sugar for optimal eating quality. Carotenoid content also increases in storage, making the butternut squash more nutritious and visually appealing after storage for a couple months. To accelerate maturity and increase sweetness, Loy found that butternuts cured at warm temperatures (up to 85 F) for two weeks develop acceptable levels of sugars. C. moschata varieties can often store for at least four to six months.
Higgins, G. and R. Hazzard. 2022. “Pumpkin & Winter Squash Harvest, Curing & Storage.” UMass Extension Vegetable Notes, Volume 34: Number 18, August 18, 2022.
Johnny’s Selected Seeds. 2022. “Eating Quality in Winter Squash & Edible Pumpkins.” Accessed August 24, 2022.
Loy, Brent. “Maximizing Yield and Eating Quality in Winter Squash – A Grower’s Paradox.” Department of Biological Sciences, University of New Hampshire.
Sideman, Eric. “Winter Squash Looks Ready, Should I Harvest?” MOFGA Pest Report, August 14, 2014.