Ever wonder why some plants are hardy enough to grow in our climate and others aren’t? The biology would take much more space than I have and require many more words than you want to read. So, I’ll give you the simplified version.
Hardy plants have evolved methods for acclimating to the pending cold weather. It isn’t much different for us humans. Have you ever had friends or relatives visit from the south in the middle of winter and feel cold all during their visits? Meanwhile, you feel perfectly comfortable. That’s because your body spent autumn acclimating to the coming winter. Your visitors hadn’t acclimated. They stepped off the plane into a Western New York winter when their bodies were used to balmy temperatures.
If you go south in winter, you’ll notice that most trees and shrubs that are deciduous here still have their leaves in the subtropics. Our lower nighttime temperatures and shorter days triggered a series of changes in plants that began their preparation for winter. The changing foliage color was the most obvious. However, other changes were going on inside the plants even before they defoliated.
Deciduous trees and shrubs made lots of food through photosynthesis and stored it in the roots, trunk and branches. In addition to sugar, this food includes an amino acid that lowers the freezing point of the water inside the cells. Even though woody plants can grow tall and hold up buildings, their cells are mostly water and they have to keep that from freezing and bursting their cells. Some northern tree species actually produce a special protein, called “anti-freeze” protein, that prevents the water inside the cell from freezing. The tree pumps this protein into the space around the cells to protect them.
Evergreens, particularly conifers, are northern trees that have adapted to surviving our cold winters without losing their needles. Their bodily functions slow down considerably, but their biggest problem is desiccation, which is why I advocate applying an anti-desiccant when preparing for winter.
Nature prepares evergreens for their leaf retention role in several ways. Needles are actually tightly rolled leaves. The shape enables more water to be retained. Also, evergreens produce a thick, waxy coating – kind of like their own anti-desiccant – on their needles. This works fine, except when high winds blow the transpired water droplets off the leaves. This is when additional anti-desiccant is needed.
Besides reabsorbing transpired water, evergreens also depend on snow for some of the water they need for photosynthesis. Some also are protected by producing the anti-freeze protein mentioned above.
Broadleaf evergreens have their own survival mechanism. The leaves curl to protect them against damage from freeze-thaw cycles. Leaves also droop to protect them from cold damage and too much sun. If this sounds far fetched, check out your rhododendrons. The leaves look like they are unhealthy, but are really just doing their job.
Herbaceous plants have other survival mechanisms. Annuals produce seed before the end of the plants’ lifecycle. Seeds are better equipped to survive the winter than the plants. Herbaceous perennials are able to store food in their roots and extensions of their roots. The tulip bulb is a good example of this.
There you have the short course in why some plants can survive in our climate and others can’t. It fascinates me how they are able to begin their natural processes by climatic factors, factors that also subliminally influence our preparations for winter and those of wildlife. This synergy is the definition of the term that we use and misuse so much in society today – ecology.
Great simple answer.