What’s a cucurbit? It’s any plant that’s in the Cucurbitaceae plant family. You eat them frequently and very likely grow them. This is the plant family that includes zucchini, summer and winter squash, pumpkin, cucumber, watermelon, cantaloupe and other sweet melons, and gourd.
They all like fertile soil, and require warm soil and air temperatures right from the start. So unless you’re planting in a hoophouse or greenhouse, don’t plant seeds or starts until late May or early June. Zucchini and other summer squash and cucumbers are easy, relatively short-season crops (50 to 65 days from sprouting or transplanting to maturity) that can be grown even in some mountain gardens. Melons take a bit longer, and most watermelons longer still. Pumpkins and winter squash varieties take the longest to mature, ranging from 85 to 120 days to full ripeness. In the Boulder metro area, we advise planting varieties that mature in 105 days or less, and the selections we sell as seeds and plants are chosen for fewer days to maturity.
We get questions about why the first few fruits on a zucchini or other cucurbit turn yellow and fall off instead of developing. This indicates a either a lack of effective pollinators, or even more likely, the lack of male flowers with fresh pollen available at the same time that the female flower is open. This often happens when there are only one or two plants of the variety being grown in the garden. We sell our cucurbit starts planted two to each pot. These are meant to grow together in the same planting hole. The same idea should be followed when planting and thinning cucurbit seeds; we recommend sowing at least 4 seeds per ‘hill’ (this is not a literal hill, but a slightly raised ‘platform’ about 3-4” above the surrounding soil so seeds don’t rot, with a lip of soil around it to keep water from running off). Once the seedlings have emerged, thin them to 2 to 4 per ‘hill’.
Cucurbits all bear separate male and female flowers on the same plant. Here are some photos of Kabocha squash flowers to help you recognize the sex of a flower. Probably the easiest way to tell a female flower is by looking at the underside of the blossom, where it attaches to the flower stem. If the flower is female, it will have a little miniature fruit in the shape of the squash, melon, or cucumber it will grow up to be, at the base of the blossom. This is the ovary that, when the flower is successfully pollinated, will expand into a fruit containing seeds. Compare this with the underside of a male flower, where the flower attaches directly to the flower stem. If you’re looking at the inside of the flower, it can be harder to distinguish male from female. Both sexes of Kabocha squash flowers are shown here, with the male stigmas and anthers forming a simple column in the throat of the blossom, while the female has a stigma that’s divided into many rounded parts with lots of surface area to receive pollen brought to it by bees. This photo shows a native Squash Bee delving into the nectaries at the base of the
flower. She has squeezed past the sticky stigmas to get there, and in so doing, has deposited pollen onto them.
To address the possibility of a lack of pollinators, increase the diversity of flowering plants in and around your vegetable garden in order to support a diverse population of native bees and honeybees. Many beautiful annuals and perennials, native and non-native, will bring pollinators and beneficial insects to do a lot of good work for you.
A common issue with cucurbit plants is powdery mildew fungus. This appears just as it sounds – like a coating of white flour on the foliage. Left unchecked, leaves and whole plants can die. Our cool nights create conditions favorable to powdery mildew. Using drip irrigation and mulch (straw or shredded leaves) and watering early in the day (before 10 am) should help prevent powdery mildew. Planting cucurbits in a location where they will have good air circulation and not be crowded is important, too. If powdery mildew is infecting your cucurbits you can spray plants with an organic, OMRI certified, non-toxic fungicide like our Sierra Natural Science ‘Disease & Fungal Control’. If you can apply it preventatively before symptoms appear, even better. The product we used to carry, ‘Green Cure’ (no longer available) was potassium bicarbonate, and the Sierra product is made with a potassium salt that they say is even more effective.
We hope you’ll have a successful and delicious garden this year!