soil temps - bulbs
Wed, 20 Nov 2013 12:45:29 PST
Gastil posits the question about chilling bulbs in a Mediterranean climate, if I understood the question correctly.

Before anyone can provide a definitive answer it is essential to know {a] which genus or genera are involved; and [b] within which species is involved.
From what little I have learned in terms of the genus Lilium species range from high altitude alpine taxa such as L. lophophorum and L. fargessii downhill
to temperate levels and low laying taxa in e.g. southern Japan such as L. nobilissimum and L. alexandrae. Then spin the dice and you have semi arid steppe 
species and Mediterranean ones. The latter I understand are best suited, if non native, to California and Oregon. 

Without in any way meaning to be facetious, how long is a piece of string. Gastil needs to tell PBS members and lurckers which genera he is perhaps most
interested in in order to get close to the answers he seeks. Many bulb genera from the northern hemisphere need winter rest periods, some longer than others
and during the Autumn in Lilium for example the bulb is producing the nascent flower bud for the following Spring, no rest no flower.

Chad Schroter rightly mentions Fruit trees as a possible temperature analogy, as a former Forest Consultant we had an interesting light bulb moment when
the onset on new tree growth in the Spring was discovered to be a function of the heat sum, in Boreal and north Temperate forests, ( 8 degrees C x Y days 
per species / latitude) With this in mind as David Pilling at c. 54 degrees North correctly points out we are at a perception of a disadvantage and further north 
I am, e.g. at close to 58 degrees North, roughly equal to your American USDA climate zones, very roughly @ 6A. it is quite challenging. Our summer days are exceptionally long with almost no darkness, we know it as ' the Summer Dim ' and right now at 15:30 it is effectively dark for a couple of months with daylight 
effective only from c. 08:30 hours. Gastil is conversely at a disadvantage because there is so little variability from Spring through to Autumn.

Pursuing the Heat Sum process I have been calibrating the Heat Sum requirement needed for initiation of growth in some 95 species in the genus Lilium and 
they even, within a species grown from seed provenances north - south and / or uphill - downhill, there are exhibited in several species a clear cline in the timing 
of the onset of growth e.g. L. martagon which has a huge geographical distribution, the greatest of any single lily species across incredible vegetation types and 
climate from Maritime to sub Arctic whereas amongst species with micro distributions such as L. pomponium there is effectively no change year on year.

To conclude, apart from the obvious parameters such as aspects, altitudes, soil types - sands across to clays, soil temperatures are immensely variable, damp soils, soils facing North, etc, etc, ad infinitum make it very hard to offer any meaningful advice except to say below 12 - 15 centimetres in depth soils are rarely going to be cooler in the sense Gastil would appear to need. Cultivating bulbs such as Lilium, Tulipa, Crocus, Narcissus, Galanthus, etc which all require a winter rest period seems to be something of a challenge to say the least but hell Gastil go for it, whatever it is, why not try and surprise us all. Iain

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