The fruit stalk is the answer
The stalk that attaches the fruit to the vine is called the peduncle, and its shape is the most reliable species marker in the group. It is worth photographing on its own, because it is the feature that settles an identification.
A hard, angular, five-sided stalk with a flared base belongs to Cucurbita pepo. A soft, round, spongy stalk that stays green and does not flare belongs to Cucurbita maxima. A hard, five-angled stalk that flares widely where it meets the fruit belongs to Cucurbita moschata.
The four main species compared
| Species | Fruit stalk | Vine stem | Typical varieties |
|---|---|---|---|
| Cucurbita pepo | Hard, angular, five-sided, flared base | Angular with prickly hairs | Zucchini, spaghetti squash, acorn, delicata, most carving pumpkins |
| Cucurbita maxima | Soft, round, spongy, stays green | Round, soft, not prickly | Queensland Blue, Hubbard, Delica, Jarrahdale, buttercup |
| Cucurbita moschata | Hard, five-angled, flares widely | Five-angled, flared at leaf joints | Butternut, tromboncino, Seminole pumpkin |
| Cucurbita argyrosperma | Ridged with a corky texture | Round and soft | Cushaw, silver-seed pumpkin |
Why the species matters in the garden
Squash only cross-pollinate within a species. A zucchini will not cross with a butternut, because they are different species. But a zucchini will absolutely cross with a spaghetti squash, and a Queensland Blue will cross with a Hubbard.
That matters for seed saving. If you keep seed from a bed where two varieties of the same species grew together, the offspring will be a random hybrid rather than the variety you planted. If you want true seed, grow only one variety per species, or hand-pollinate and bag the flowers.
Reading the fruit and the seed
- Seed shape is a useful backup. Cucurbita pepo seeds have a distinct raised rim around the edge. Cucurbita maxima seeds are smooth and rounded with a blunt tip.
- Flesh texture follows the species. Cucurbita maxima fruit is dense, dry and sweet, which is why the best roasting pumpkins are in this group.
- Cucurbita moschata flesh is smooth and firm, which is why butternut holds its shape so well in a curry.
- Cucurbita pepo covers the summer squash as well as the winter ones, which is why the group feels so inconsistent.
- Skin hardness is not a species feature. Both summer and winter types exist within pepo.
The bitterness warning that matters
Occasionally a squash, usually a home-grown one, will taste intensely bitter. That bitterness is cucurbitacin, a compound the plant produces as a defence, and at high levels it causes violent stomach cramps and diarrhoea.
It is not a flavour to cook around. If a squash tastes bitter, throw the whole fruit away and do not use the seed from that plant. Commercial varieties are bred to be low in cucurbitacin, but a chance cross with an ornamental gourd can bring it back.
Squash identification is unusual because the shape of the fruit tells you almost nothing. Butternut, zucchini, spaghetti squash and the giant pumpkins all belong to a handful of species, and within each species the fruits vary wildly. A Queensland Blue and a Hubbard are both Cucurbita maxima; a zucchini and a spaghetti squash are both Cucurbita pepo.
What actually identifies the species is the stem and the stalk. Once you know those two features, the whole group falls into place, and you can predict how the plants will cross-pollinate and how the fruit should be stored and eaten.
Frequently Asked Questions
The words are not botanical. Pumpkin usually means a round, orange, thick-fleshed winter squash, and squash covers the whole group including summer types. To identify the plant properly you need the species, which the fruit stalk gives you.