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. Also, consult Sakai references.
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.