Alpine houses and frames
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Although most alpine plants are very
frost hardy they often suffer if exposed to prolonged periods of wet weather
late autumn to early spring. Often such plants have evolved to spend the winter
in a fully dormant state under an insulating blanket of snow. You can eliminate
the excessive moisture and very low temperatures by growing these plants in a
greenhouse.
An alpine house is one of the simplest
greenhouses to operate and ultimately one of the most satisfying. It is an
unheated greenhouse that is very well ventilated. The objective is simply to
prevent the plants becoming too wet at the crown, not to raise the temperature
or force the plants into flower. Alpines are used to hard conditions, too
gentle an environment leads to rank growth and shortens their lives. The vents
should be open even in winter and the house should not be insulated except in
very cold areas.
The plants are usually grown in pots on
benches. The pots should be topped with a layer of fine gravel to keep the crowns
dry. The benches are usually constructed with a frame on top that is filled
with gravel to a depth of about 15 cm. This is sufficient to allow small pots
to be plunged to their rims in the gravel. This is important because alpines
resent the wide fluctuations in soil temperature that can occur in pots that
are not well insulated.
Benches of this type become very heavy so
it vital that they are sturdily built. If this presents difficulties it is
sometimes possible to get by with a shallower layer of gravel but more careful
attention will need to be paid to watering and ventilation.
Although plastic pots have almost
entirely replaced clay pots in everyday use the old clay pots are very useful
in the alpine house. Their natural porosity allows them to absorb enough
moisture from the plunging gravel that it is often unnecessary to individually
water the plants over winter. The gravel between the pots can be watered and
the pots left to absorb the moisture. This helps to keep the crowns of the
plants dry while maintaining soil moisture.
When spring arrives many of the less
fussy plants can be moved outdoors to a rockery, alpine troughs or similar
situations. Some, however, are best kept in the alpine house year round. These
are the plants, such as Draba and some succulents, that resent excessive
crown moisture at any time. Some summer dormant bulbs, such as many of the Oxalis
species are also best given alpine house treatment in colder areas.
If you can’t afford an alpine house, or
don’t have the space, frames are a sensible alternative. They give your plants
many of the benefits of a greenhouse but of course they lack the one great
feature of a full size house: you can’t fit in there as well.
Permanent frames
A cold frame is simply a box, usually wooden, that slopes towards the front. A glazed or plastic covered lid completes the unit. A few centimetres of gravel in the base of the box ensures good drainage. It is effectively a miniature greenhouse.
A heated frame is exactly the same except
that it’s heated. In years gone by a compost made from hot decomposing dung was
used. Thankfully we now have electricity. Cold frames are more practical if you
don’t have a greenhouse but heated frames are a useful addition to an existing
greenhouse. The heating cables can be wired into the greenhouse power supply.
Frames need insulating in winter. The
usual method is a hessian cover that is rolled up when not in use during the
day. Double glazed or double skinned lids are also helpful. Cold frames are
difficult to keep frost-free in areas that experience air temperatures below
-4°C but they offer enough protection to significantly advance the date at
which young seedlings or struck cuttings can be moved from a more sheltered
environment for hardening off. They also allow autumn struck cuttings to be
moved out later than would otherwise be possible.
Portable frames
Portable growing frames, usually made of
lightweight pipework or strong wire covered with plastic film are useful for
giving crops an early start. They can be moved around the garden to wherever
they are most needed. More substantial structures that can be easily dismantled
and moved around can be used for longer term protection. A portable frame is
also useful for keeping birds off tender young plants, such as lettuces.
Lightweight structures with a large surface area are very prone to wind damage so make sure that any large frames are firmly tethered. Tying the corners of the cover to stakes driven into the ground is the usual method.
Cloches are the ultimate in portability but they offer the least protection. They are useful for covering early season crops but should not be relied on for anything but the most minimal frost protection.
Copyright Geoff Bryant
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