Light Requirements for Dormant Evergreens

by Andy Walsh

Introduction

This article was originally a post to the Internet Bonsai Club Mail List. It has been edited slightly to make it more readable in this context. Andy Walsh is a frequent poster of good sound scientifc advice for growers of bonsai.

This is Andy's response to a question regarding the necessity of light for dormant evergreens.
Brent

I came across an interesting factoid in my readings that I thought some of the members may be interested in.

Many people have alluded to the idea that evergreens continue to photosynthesize in the winter. It would seem to many that this would be so since the evergreens hold on to their leaves through the winter. The rate of photosynthesis is temperature dependent and I have always assumed that at very low temperatures this rate would be so low as to be undetectable and basically at an apparent standstill. However, in the book "Plants and Temperature" the authors discussed some research on the effects of low temperatures on the various photosynthetic activities, and on the photosynthetic apparatus itself. It appears that at very low temperatures photosynthesis is still plodding along at a rate that can be measured.

The interesting thing that they mentioned was that at very low temperatures the rate of photosynthesis is so low that the relative level of light is therefore high (not being absorbed) in the cells and this excess light can damage the photosynthetic apparatus. This effect is known as "photoinhibition". They write that "Even if the temperature stays above the limit of frost tolerance, the thylakoids may be damaged when cells in the frozen state are exposed to high light. In conifer needles, such photoinhibition appears to be a major factor of winter damage but usually can be repaired in the growing season."

I think these tidbits offers some guidance on what might be best for wintering evergreens (conifers at least).

It doesn't seem to me that an evergreen benefits much from any amount of photosynthesis in winter so I don't really think wintering in the dark or near dark in something like a coldframe is much of a concern. However, for those who leave their trees outside through the winter, it seems that keeping evergreens out in the open where direct sunlight can hit them, or even possibly stored under plastic type coverings, might be something to worry about.

From this information I would conclude that when wintering evergreens, some light is probably good but not any direct sunlight.

Andy

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