Hardiness - Cold hardiness varies greatly with the bulb species, the
exposure temperature and its duration, and the precise conditions in which the
bulbs are either packed or planted. In the latter situation, the depth
of planting, moisture content of the soil, and the presence of a mulch affect
the survival of the bulb.
In addition, the species and/or cultivar can be a factor. Thus, hardiness
is very complex. Traditionally, bulbs have been classified as "Hardy",
"Semi-Hardy", or "Tender". In addition, the usage has been largely confined
to bulbs planted outdoors. In reviewing the literature on this subject,
De Hertogh and Le Nard (Chapter 6, The Physiology of Flower Bulbs, 1993, Elsevier
Science Publishers, Amsterdam) have proposed the following hardiness classification
for flower bulbs.
It was proposed that these classifications will be more functional because they define upper and lower temperature hardiness limits and these are directly related to optimal growth and development responses.