soil temperature variation with depth in a bulb garden
Jane McGary (Fri, 22 Nov 2013 17:11:26 PST)
Boyce wrote,
To increase the probability of providing soil temperatures cool enough to
perennialize some of the Mediterranean species of tulips try planting them
on the north side of a structure and/or in the cracks between large paving
stones. Photo's taken of Tulipa species in the wild in Central Asia
indicate a number of them grow in really rocky habitats. I thought this
was only to reduce predation by herbivores but there is a cooling component
as well. Both microclimates (north side of a structure and use of large
stones) are amazingly efficient at reducing the temperature of the soil.
A method I used with success in my former garden was as follows.
After planting bulbs on a raised bed with a gritty soil incorporating
coarse, sharp sand with the native gritty clay loam, I spread a
single close layer of basalt rocks around the size of medium to small
apples (I had an infinite supply of these in the rocky soil there).
Then I spread a layer of the coarse sand over the rocks and
top-dressed it with pea gravel. Low-growing rock plants and subjects
such as Acantholimon went in as my seedlings got enough size. Many
plants that I thought might not be cold-hardy there grew well for
many years. I haven't done this in the new garden because I didn't
bring small rocks (I brought a lot of large ones for rock garden
construction), but it's a good way to use up a size of rocks that
aren't good for visible construction.
Jane McGary
Portland, Oregon, USA