One of the things I found fascinating when first learning a little horticulture out of necessity was chill hours requirements of plants. I know, I know; it’s a pretty boring subject unless you are one of the few people dead set on raising your own peaches or apples in the backyard. And blueberry bushes and muscadine vines do the chill hours thing.

But quite a few of our favorite non-edibles also have to stockpile hours of cold temperature in order to initiate new spring growth. For the ones that bloom before putting on new leaves, i.e. azaleas, forsythia as well as the fruit trees, chill hours set things up for blooming. For others, such as deciduous trees like oaks, initiation of leaf buds comes with warm weather once the accumulated chill hours are right.

How about roses? Here we can learn something about the climate origin of plants. Three hundred years ago mankind put roses in two categories. There were the European roses and the China roses. To this day the pure China varieties do not have a chill hours requirement but the pure European ones do.

However, most modern rose varieties are some combination of European and China DNA with three centuries of rose breeding. I suppose a chill requirement of some sort is in modern roses, but it really does not matter. Just plant the ones that can withstand heat, humidity and fungi and let Nature take care of chill hours.

Chill hours are defined as the number of hours a plant is exposed to temperatures between 32 degrees and 45 degrees Fahrenheit. Below freezing and above 45 do not count. The minimum hours required varies a lot, not only among plant species, but also among varieties of the same species.

Once a plant has its chill hours need met, it is set to move to the next stage, be it bloom or leaf out, as soon as it gets enough warm weather. And that’s why chill hours matter. If the chill deal is met too early, a February warm spell can cause plants to bloom out too early only to be killed by a late winter hard freeze.

Killed off blooms cannot be replaced. Freeze-killed leaves can and will be. With plants we grow primarily for their pretty spring blooming, a shortage of chill hours might be the culprit for an occasional weaker-than-normal bloom. Then again, a last summer’s drought might bear the blame.

The way to avoid chill hours disappointments is to stick with plant varieties recommended for this area. And then don’t try to figure out chill hours for the ornamentals. With fruit trees, the varieties that need 550 to 700 chill hours are our most dependable.

The 1,000 hour varieties that work in Indiana will just not bloom and fruit here most years. Conversely, don’t plant the coastal types subject to bloom after 300 or so hours. If you do you will be whining about a freeze killing blooms and/or fruit most every year.

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Terry Rector writes for the Warren County Soil and Water Conservation District, 601-636-7679 ext. 3.