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 available foremost from your supplier, and you
should have close consultation as far as schedul-
ing crops ad cultivars for each intended forcing
period. A large volume of information is on the
website of the Flower Bulb Research Program
website (www.flowerbulbs.cornell.edu), and more
is on the way.
Diagnosis of cooling problems changes through-
out the season. Obviously, early season forcing,
where plants are being pushed to flower as early
as possible, is when you see evidence of lack of
cooling, and symptoms may vary from green tips
of hyacinth flowers, to white/tan tips of tulip pet-
als, to excessively short stems that flower inside
the leaves (tulips). Such problems can also occur
with later forcing, but are less common. In late
season forcing, one can begin to see problems
from too much cooling (rapid, weak and too tall
growth, lack of stem and leaf strength, small flow-
ers).
What is “too much” cooling? In North America,
we tend to plant bulbs in a rather narrow window
in the fall, and put it all into the cooler. Crops for
mid-season forcing (e.g. an early Easter) might
receive perfect cooling, whereas late Easters and
certainly Mother’s Day forcings are usually over
cooled. While PGR use can increasingly help
with excessive growth from over cooling (see be-
low), the rapid plant development can cause chal-
lenges all throughout the marketing chain.
Greenhouse Temperatures
A commonly used greenhouse temperature for
mid-season forcing is 63F (17C) day and night.
Most crops can tolerate significant deviation from
this. Early crops are usually grown somewhat
warmer to speed development for the earliest
market, whereas lower temperatures can reduce
growth rate and delay flowers (this is not always
possible with late crops, however).
Five degree tulips, which are given all cold before
planting, must be grown significant cooler (50-55F
constant) as a way of reducing shoot growth fore a root system is developed. See above un-
der “soil temperature” for more information on
planting 5-degree tulips.