Most years swedish aspen turn yellow, much like trembling aspen. A couple times I've seen rose tints in the yellow. But look at my collection this year. Orange, burgundy, bright red? Sometimes all on the same tree? Not all my aspen are affected this way. Overview as to the physiology of different colours. Can this be done deliberately? Edmonton fall colours are primarily yellow. I'd love to be able to do this on command, or at least make it more likely. .
A Sierra Club desk calendar once had a photo of a quite red aspen in the midst of a patch of typical yellow ones.
Given that it's the sierra club, almost certainly trembling aspen. Which also means the possibility of them not being clones. But as far as I know all the horticultural swedish aspen are clones of a single tree, and so genetically identical.
Responding to my own post: The fall colours I posted are not unusual. I made the rash assumption that since our native trembling aspen was almost invariably yellow to yellow brown, that swedish aspen was going to be similar. My bad. I have since found pictures of other red ones. The mix of colours I have right now is a bit more than is usual, but not outrageously so. Let me re frame the question: Why does the same tree colour differently in the fall in different years?
From what I understand, color can be and is affected by temperature and moisture. If days are warm and nights are cool, sugar is produced in leaves that is prevented from flowing back out of them into branches and trunk. This situation causes anthocyanins to be produced, which create red hues in leaves. Carotenoids, which produce yellow, are not so affected by temperature and remain fairly constant. Drought can delay color: warm wet weather will dim color: hard frost will kill the leaves and cause them to drop. So if you like robust fall color, you should wish for a warm, wet spring, summer that is not too dry or hot, and fall that has warm sunny days and cool nights.
That may explain it. These guys got a shot of granular fertilizer in early August, and were on an automatic waterer that put a gallon of water on each one. Alberta's climate is normally one with day/night differentials of 10-15 C. We've had one light frost. Generally the fall colour of swedish aspen here is not remarkable. Most years they are yellow, with tinges of pink on the edges, or on individual twigs. Some years the burgundy-russet is moderately common. While I've found internet examples of the brighter reds, I hadn't seen them locally before. Our Augusts are usually quite dry. I'll have to watch and see of ones that don't get watered as much have less intense colours.
I have an overgrown forsythia that has kind of grown against a tree and the branches hang over my garden. Summers are generally dry here. When I water, I have sprayed the branches of this forsythia. I noted the past two years that the branches that receive the spray seem to colour more in the fall. This is a purple tinge. I wonder if it is the cooling of the leaves that causes this and if this could be done with other plants. Spraying water on leaves can cause other problems though.
Partly purple fall color is typical with these. Possibly more sun-exposed foliage is where the purple concentrates, I am not sure.
