Pilchuck Pollinator

Running Hot and Cold — cont.

We have all heard about not pruning some shrubs in late summer and fall. This is because some plants respond to pruning with new growth which is quite tender, and is easily killed since it has not become acclimated to the cold temperatures.

Did we mention anything about provenance? Provenance, in lay terms, means where did the parents come from? Just like parents and their kids, offspring bear some resemblance to the parents. Plants are similar, in that seed taken from a tree in Minnesota will be more cold hardy than seed taken from the same type of tree in Florida. Conversely the plants from the Minnesota seed source might never break dormancy in Florida, due to the lack of cold.

Many reference books may say that red maples are hardy from zone 2-9. Granted, there may be red maples growing in both areas, but to interchange seed from each area would most likely prove disastrous. This problem is particularly dramatic in woody plants that are grown from seed. It is also usually the most important at the extremes of the zone for each plant. In the case that we mentioned, the gardeners in zone 2,3 and 8,9 would need to be the most cautious of the provenance involved.

Cultivars or vegetatively propagated identical plants (clones) keep the same hardiness regardless of where they are produced commercially. In other words Hemerocallis 'Stella D'oro' has the same hardiness whether it is produced in Florida or Chicago. The hardiness of a plant is based on the origin of the original genes, not where we as humans move the plants.

Reference books and most plant catalogs have completely neglected the effects of heat on plants. Many plants from the north are not able to withstand southern hot summers. The American Horticulture Society has recently devised a "heat map", but while a good idea, it confuses the issue with two sets of number...one for cold and one for warmth. Heat hardiness is an issue of night temperatures and humidity as well as actually heat tolerance.

 Another wrench enters the picture when we talk about night temperatures compared to day temperatures. In many cases, the culprit is not just the high day temperatures but also the high night temperatures. During the day, plants store up energy produced through photosynthesis. If the nights are cool, the energy goes into growth of the plant. If the nights are too warm, the energy is burned up by the plant. Many plants, due to their with metabolism derived in a cool night climate are not hardy in areas, simply because of their warm nights temperatures.

There is still another factor in heat hardiness that we have overlooked...one of heat dormancy. A hosta, for example will not grow well in parts of Florida, parts of zone 9, and 10. The problem here is that temperature do not drop low enough in the winter time for the plant to grow completely dormant. Many plants, both perennial and woody must have a specific dormant period in order to start growth again in the spring. A hosta must be exposed to at least one month of temperatures below 40 degrees. If this temperature requirement is not met, the plant will begin to decline in the spring, or in the case of some trees, will never resprout in the spring until the dormancy requirement has been satisfied.

If I have completely frightened anyone from ever gardening again, I apologize. What I hope you will realize is that growing plants can be very complex. Don't be frustrated when a new plant dies, and certainly don't give up. There are many factors at work in determining whether a plants survives or dies, and you play only a very small part. After all, remember that gardeners are like doctors...they bury their mistakes.