The
good news is that days are getting longer, but the bad news of course,
is there are three more months of winter.
Given that the average annual temperature is usually just a degree
or two above or below the average for all years, and that we had a very
warm fall: it might be very cold during the next three months.
Weather extremes are the real challenge for orchid growers. One night below 32ºF or above 120ºF and the
rest of the year does not matter. A
heating or cooling system must be able to handle the worst possible
condition, otherwise you run the risk of loosing your orchid collection.
When
does the worst scenario occur? When
you are least prepared. For
me, it was waking up in the middle of the night and realizing that the
house was cold and very dark. The
power was out on a record cold night.
Of course, gas heaters with electric fans and thermostats were
also not working and the greenhouse was already down to 35ºF.
A kerosene heater and Coleman cooking stove saved the day.
Even though the greenhouse temperature was below freezing for
10 minutes or so, no plants died. Why? Fortunately, plants contain lots of water that
holds heat, which means that a plant has a certain amount of residual
heat.
Orchids
are also more tolerant of extremes if they return to normal temperatures
quickly and if they are in good health.
The morning sun hitting a plant leaf quickly warms the leaf,
just as the sun quickly warms your car.
Phals should be kept at a minimum 65ºF at night in winter.
On extremely cold nights my greenhouse may approach 60ºF, but
there is rarely any direct or indirect damage as long as the daytime
temperature gets back to normal quickly.
On very cold nights followed by gloomy days where the greenhouse
remains at the minimum 65ºF, there are often phals that drop buds and
bacterial rots appear in the next week or so.
These are just some of the sub lethal effects that come when
a phal’s internal temperature gets too low.
Each type of orchid has a minimum temperature below which it
stops growing and becomes vulnerable to diseases.
Many
greenhouse heaters are equipped with day/night thermostats that allow
you to set a different greenhouse temperature for night and daytime. The change in temperature during the day is
also important. While this feature
may only be used a few times a winter, it can make a huge difference
in the health of your plants.
This
time of year when greenhouses are closed is also a time to carefully
watch for disease. Besides maintaining
the proper temperature, avoiding overly high humidity and maintaining
good air movement are critical. Cold
plants will condense moisture on both leaves and flowers leading to
Botrytis (small brown spots on flowers) and various fungal rots.
Humidity levels at 70-80% at night are ideal, but avoiding 100%
humidity is critical. It might
seem impossible to keep humidity lower than 100% on very cold nights. Gas heaters tend to lower humidity on cold nights because the air
burned is replaced by cold air from outside that is typically very low
in humidity.
If
you use electric heat or if the humidity outside is very high it can
be difficult to keep humidity below 100%.
Rapid air movement is the only solution.
Avoid watering during these periods.
Commercial growers often use preemptive spraying, a costly alternative.
Good greenhouse hygiene is now more important than ever and cannot
be emphasized enough. Remove dead leaves and flowers from plants,
benches, and floors. You may
even find that spraying Physan on floors and benches will help. On warm
days, turn on ventilation fans and exchange air inside the greenhouse
as much as possible with fresh air.
Most growers that report constant problems with disease fail
to follow these basic tenants of good culture.
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