This family is represented by plants whose flowers are generally unisexual, i.e. with male and female flowers on the same plant. The male flowers, at the tip of a long peduncle, fertilize the female flowers, in which the ovule is already in the shape of a miniature fruit. This means that different plants must be cross-pollinated, either naturally (by insects or the wind) or manually.
Cucurbitaceae easily form hybrids between species, producing different cultivars. Be sure to plant any cultivar whose seeds you intend to keep well away from any other sources of Cucurbitaceae pollen.
Harvesting seeds from various cross-pollinating vegetables
Squash and pumpkins
Harvest your squash and pumpkins once they are fully ripe. The skin should be thick and not yield to pressure from a fingernail.
Leave the seeds to mature fully for at least three weeks after you harvest the plants. Then cut the fruit in two and remove the seeds, separating them from the flesh. Blot them dry and spread them out on a cookie sheet or waxed cardboard for two to three weeks.
Allow your cucumbers to become over-ripe before harvesting them – they will turn soft and dark yellow, orange or white, depending on the cultivar.
Cut them in two and use a spoon to remove the seeds and pulp. Transfer it all to a glass jar and add as much water as there is pulp, then place the jar in a dark spot for 48 hours. This will allow it to ferment, which will clean the seeds. Any imperfect seeds and the pulp will float to the surface, so you can skim them off. Rinse the remaining seeds in a sieve and dry them as you would squash or pumpkin seeds.
Harvest your melons not long after they ripen (but before they rot or get eaten by rodents!). Remove and rinse off the seeds and dry them as you would squash seeds.
N.B.: You can grow muskmelons and watermelons together and harvest their seeds without worrying about them cross-pollinating, since they belong to two different genera and species.
On the other hand, you must make sure that your neighbours are not growing them within 300 metres of your garden.
Once the pods holding the seeds have turned brown, you can cut down the plants and lay them on a sheet of fabric to catch the seeds when the siliques burst.
Keep only the main flower head and prevent any secondary shoots from developing.
Do not choose any plants that bolt, or flower prematurely.
N.B.: Broccoli must be grown one kilometre away from other members of the cabbage family, in particular different varieties of Brassica oleracea. On the other hand, you can grow other broccoli plants and pick them before they flower.
Harvest the seed pods before they open and place them in a paper bag. Choose the latest-flowering radishes.
N.B.: Radishes cross-pollinate with each other and with turnips, rutabagas, Chinese cabbage, colza (rape) and mustards. Do not allow more than one cultivar to flower, or keep them one kilometre apart.
Based on an article by Nathalie Leuenberger in Quatre-Temps magazine, Vol. 23, No.4