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This zone information is given as guidelines only.
Many things go into hardiness considerations. Wind, snow cover,
temperature, humidity in the summer and coolness of nights can all have an
effect on the hardiness of a trillium.
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Plants that are a zone 6 can often survive nicely in zone
4 if covered by deep snow or mulched with several inches of material in
the fall.
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Mulching plants can keep them from coming up too early in
the spring.
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Placing a plant on the protected side of a building can
help with wind damage in the spring. Continued wind damage year
after year will eventually kill the plant.
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Trillium can stand
very hot temperatures in the summer with adequate water, afternoon shade
and cool nights.
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Trillium are native
woodland plants and as such, they are used to a
drier season in July and August. Drier doesn't mean bone dry and
adequate water doesn't mean soggy.
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So-called "boreal"
plants, of which Trillium undulatum (T. undulatum) is an example,
have their range restricted by their inability to shut down their
respiratory/metabolic processes at night if the temperature remains above
a critical level (different for different species). If you look at the
distribution
map for T. undulatum, you'll see
that it grows in the north or in the mountains, but nowhere where summer
nights are hot. Note, in the southern states it is restricted to
mountainous areas where the nights are cool. T. erectum can hardly be described as "boreal", yet its
distribution suggests a similar pattern, although less restrictive. In
fact, it appears that almost all of the pedicillate trilliums tend to be
the same way. Very few grow in areas where summer night temperatures
remain elevated. The true T. cernuum barely makes it in Delaware, but
T. rugelii certainly is less sensitive in that regard.
It's becoming more and more apparent that summer temperatures are as
(or more) important as winter lows in assessing plant hardiness. And the
former is much more difficult for gardeners to manage. It's relatively
easy to extend the winter hardiness of a plant by mulching or location,
but, short of enclosing and air-conditioning your garden, hot summer
nights are invincible!
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Micro Zones...
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