Apple varieties of different maturity requiring different chill hours. Most of the first three grew naturally in much colder climates than Australia and so need a lot of winter chill to break dormancy. The last species is more from Southern China and some of these do not require so much chill. Varieties in Australia are mostly P. communis, pyrifolia and bretschneideri or hybrid crosses. Some of these have European pear ancestry or are European pear crossed with another species, which may have given the new variety a low chill characteristic.
Pear varieties with certain species ancestry and different chill requirements in Australia.
The chilling requirement of a rootstock can have an effect on the chilling requirement of the variety. Most of the clonal apple rootstocks used commercially were selected and bred in much colder climes, so how they work in warm districts is not been fully tested. Granny Smith seedlings are not so high chill may be used but the tree vigour tends to be too vigorous. Pear rootstocks have more choice as P. calleryana rootstock grown all over Australia. Although varieties needing high chill can be grown in warmer districts with the aid of chemicals like Cyanamide, the use of suitable low chill varieties of apples, pears and nashi can be more effective.