Classification based on the plants botany is the favored method for plant botanists, taxonomists and breeders. This system groups the plants into genera including class, family, genus, species and subspecies. Although this information is extremely useful to those interested in plant breeding and improving the plant lines it is often of little use to growers because the cultural requirements of plants can vary greatly between genera. Some plants do however have similar cultural requirements the most notable of these is the Solanaceae and Cucurbitaceae familes. It is for this reason that when grouping plants according to their ‘cultural requirements’ horticulturists have grouped Solanaceous and Cucurbit vegetables in their own distinct groupings.
For the grower this is probability the most practical system of classification as it is based on essential methods of culture. That is if a crop has similar climatic requirements and cultivation requirements then they are group together. This grouping can does often mean that crops group under the same heading may be botanically divergent. Perennial vegetables – asparagus, rhubarb, artichoke
For growers the classification of vegetable crops by similarities in their temperature requirements (Thermo-classification) or by their ability to withstand frost (Hardy ness classification) assists growers in planning their planting schedules. However hardy only refers to frost and some hardy vegetables will not withstand summer drought, while some tender crops do not thrive in cool weather even when there is no frost.
Not unlike hardyness thermo-classification is also based on temperate. The difference is that crops are classified according to their optimum temperature requirements and are divided into two groups either as cool temperature or warm temperature crops. Cool temperature crops have an optimum temperature of 12-20oC and are able go as low as 1oC, no mention of their ability to withstand frost are given in this classification. Warm temperature crops have an optimum of 18-20 oC
Cool temperature crops are as follows: Green peas, lettuce, cabbage, cauliflower, broccoli, onions and spinach are all ‘cold season ’ crops. ‘Warm season’ crops are; Beans, cucumber, eggplant, okra, pepper, summer squash, watermelon, and tomatoes. This system of classification gives the grower an indication of the crops ability to withstand high temperatures but nothing in regards to frost. In order to have a clear idea on which crop to use when planning a planting schedule it is advised that the grower use the ‘hardyness classification’ in conjunction with the thermo-classification.