116 – II.2. SQUASHES, PUMPKINS, ZUCCHINIS, GOURDS (CURCURBITA SPECIES)
Agricultural practices
The cultivated Cucurbita are frost sensitive and need frost-free growing periods of
four to five months. Temperatures of 20-35°C are ideal for growth. The Curcurbita
can be grown on a wide range of soil types. They prefer a soil pH between 6.0 and 6.5,
although they will tolerate both slightly acidic and slightly alkaline soils. As roots
can penetrate up to a metre into the soil, a well-drained soil is preferred. Cucurbita also
have feeder roots close to the surface. Roots can grow to about the same spread as the
vines. The Cucurbita are sensitive to soil salinity. They are also susceptible to herbicide
damage, and this susceptibility would suggest that care should be taken with herbicide
use, or that herbicides can be used for control. The cultivated Cucurbita are usually
established by direct sowing of seed, although seeds can be sprouted in containers and
seedlings transplanted to the field when about 10 cm high. Seedling transplant is
particularly indicated if the growing season is short. Cucurbita are insect pollinated
and require bees for pollination. Inadequate pollination results in poor fruit shape and
blossom drop.
Cucurbita fruit develop rapidly after the flower closes, and fruit eaten at the immature
stage (e.g. zucchini) must be harvested before the rind begins to harden. For those types
that are eaten after the fruit fully matures (e.g. pumpkin), three to four months are
generally required to mature a crop to this stage. At this stage the fruit is hard and
imperious to scratching; the fruit is removed from the vine with a portion of the stem
attached. The mature fruit can be kept in long-term storage (e.g. four to six months)
if the fruit is properly ripened and cured. Curing hardens the shell, heals superficial
wounds, reduces the water content of the fruit and improves the quality of the flesh.
Fruits can be cured by leaving them in the field in warm and dry conditions for ten days
to two weeks or by keeping them inside at room temperature for a month (OMAFRA,
2011). Table 2.4 offered phenological notes on the various Cucurbita species, including
the cultivated species, in Mexico.
Cucurbita species can express highly oxygenated, triterpene compounds called
cucurbitacins, which taste bitter to humans and can be toxic (US Environmental
Protection Agency, 1999). Cultivated Cucurbita varieties intended for consumption by
humans or domesticated animals have been bred to express low levels of cucurbitacins.
However, plants in wild populations express high levels of these substances and if a
cultivated plant is visited by bees carrying pollen from plants in a wild population, higher
levels of the toxicant can be produced in the fruit. Any resulting seeds would produce
plants with bitter fruit as bitterness is a dominant characteristic. Higher cucurbitacin
levels can also be expressed by the plant in response to stresses such as drought,
high temperatures, low soil fertility and low soil pH. Higher levels can also be expressed
in the newly emergent seedling, and by improperly cared for, harvested fruit. In addition
to producing inedible fruit, plants producing higher level of cucurbitacins can attract
phytophagous Chrysomelidae beetles and attendant pest management problems (see
below for additional information on the cucurbitacins and Chrysomelidae beetles).
Cucurbita argyrosperma
The cultivated varieties of Cucurbita argyrosperma are used in the traditional heavy
rain agricultural systems and are sown at the start of the rainy period (May-June in the
northern hemisphere) from 1 000-3 000 metres. Growth of these varieties lasts five to
seven months; the young fruit for vegetables is harvested approximately three months
after being sown, while the ripe fruit for seed is harvested between October and
