Temperature Control In The Cooler
Temperatures in the cooler are critical for several
reasons. Most important is that each cultivar has
a minimum and optimum duration of cooling for its
intended use. Bulbs that do not receive enough
cold will flower too late or not at all, and are likely
to have various flowering abnormalities (green or
bleached flower or petal tips, too short stems,
etc).
Cut tulips tend to be cooled at lower temperatures
than pot tulips because cooling at lower tempera-
tures (e.g., 35-38F vs. 45-48F) will produce tulips
with longer stems and shorter flowers. Thus, for
pot tulips, cooling on the warmer end is, in theory,
beneficial to producing a slightly shorter plant with
larger flowers.
The problem comes with root and shoot growth.
As pointed out above, excessive early root growth
leads to heaving problems in hyacinth and daffo-
dils, and increases susceptibility to Trichoderma.
Cooling the crop at a lower than typical tempera-
ture can dramatically reduce total root mass in
tulips, and can help with Trichoderma. The typi-
cal schedules and recommendations for pot tulips
are to start cooling at 48F (9C), reduce to 40F (4-
5C) when bulbs are well rooted, and the lower
temperature to 33-34F (1C) when shoot growth
reaches 1”. While these are excellent general
guidelines, if you feel your crop is putting on too
many roots, you should reduce temperature
quickly, as the lower temperature will reduce root
growth. This is also compatible with reducing
shoot growth, as one of the biggest problems is
when shoots grow up into crates stacked above,
leading to a near total loss of quality.
Optimum Cooling Time
Every cultivar has a minimum, optimum and maxi-
mum cooling duration that depends on its final
use. In general, tulips grown for cut flowers have
a longer optimum cooling duration than if the
same cultivar is used in pots. As a general rule,
spring bulbs flower faster, more uniformly and
taller as cooling duration increases.
Specific information on cooling duration should be