Some Plants Need To Chill

Big pecan trees have humble beginnings.

It is easy to snivel about the weather when it gets uncomfortable for us. The rain gets too wet. The temperature gets too cool. Even here on the west coast, without the cold of Minnesota, the heat of Arizona, the humidity of Louisiana or the rain of the west coast of Washington, we tend to think about weather by limited human standards. What we fail to consider is that many other organisms rely on a variety of weather conditions for their survival.
Deciduous plants make it obvious that they know how to deal with cool winter weather. What is not so obvious that that many deciduous plants actually need specific wintry conditions to be convinced that it really is winter. If the weather does not get cool enough, or stay cool long enough, some plants do not go dormant long enough to get the rest that they need in order to perform adequately the following spring and summer.
For example, the reason that only a few of the many different varieties of apple can be grown locally is that most have chill requirements that exceed what they get here. A chill requirement is a specific duration of cool winter weather. Only a minority of all varieties were bred for their minimal chill requirements, so that they will produce reliably even where winters are innately mild.
Besides chill requirements, some seeds like to be soaked in moist soil before they germinate the following spring. This lets them know that it is raining, like it typically does in winter. Pecans, for example, can be soaked for a while inside before sowing, bur really prefer to be out in the garden through winter, where they can tell than rain water is actually flowing past them through the soil, and the microorganisms in the soil help to break down their shells.
It is all about timing. Chill requirements get apple trees to bloom in spring, only after they were convinced that it was already winter. If they were not so specific, they might bloom after a brief cool spell in autumn, leaving their blossoms or developing fruit vulnerable to later frost. Pecans germinate and grow only after the danger of frost, but before the weather gets too dry. If they start too early, they may not survive frost. If they start too late, they may desiccate through summer.
highlight: pecan
One might surmise that a tree that is resilient enough to be the state tree of Texas is not too discriminating. If it can take the heat and humidity of the Lone Star State, it can make it anywhere! However, pecan, Carya illinoinensis, actually prefers heat and humidity, and is bored with the mild local climate. The nuts and the mess that comes with them are actually less abundant than they would be in the Gulf Coast States. The deciduous foliage is not quite so colorful in autumn.
A mature pecan tree may stay a low as fifty feet, or get twice as tall. The height is usually nearly double the width. Generous watering can cause roots to buttress and displace nearby pavement. Most local pecan trees that are intentionally planted are garden varieties that were bred for bigger pecan nuts. Seed grown trees tend to produce nuts that are nearly comparable to the nuts they were grown from. The pinnately compound leaves have nine or more leaflets that are about two or three inches long.