A number of us are discussing a common wisdom thing about not pruning your roses before the forsythia bloom or only when the buds start swelling. The implication that I will harm my roses by pruning before they start to come out of dormancy doesn't exactly make sense to me. ...the implication of what people say is that pruning in late winter, before they come out of dormancy on their own, brings them out of dormancy too early, making them more vulnerable to freezes than they would otherwise be. Is there any science to support that?
I love getting questions like this! I've heard this since I began gardening, and never stopped to wonder if it is true, and if so, WHY?
Pruning can break dormancySo I've been poking around, and it turns out that yes, pruning woody plants can cause them to break dormancy earlier. Most of the research on the topic is in grapes, but from a very different perspective than those of us in cold climates worried about late freezes. Rather, I found a lot of research on growing grapes in warm, semi-tropical climates where there isn't enough cold to break dormancy naturally. In Taiwan, is appears, grape growers can keep their vines growing without a winter by using a combination of severe pruning and plant hormone treatments. But it isn't just in grapes. I found studies of cherries, peaches, and apples with similar findings. So many woody plants are stimulated by pruning, even when they are dormant.
More susceptible to freezing?Interestingly, though, the one paper I could find that actually measured the winter hardiness of developing buds at several time points after pruning didn't find any change, so there isn't direct evidence that early pruning will lead to more damage from late freezes. That isn't to say it doesn't happen, however. Cold hardiness is notoriously hard to study because there are so many factors from moisture to time to temperature that make it very hard to recreate the real world effects of cold in the lab, so just because one group of researchers weren't able to find a difference doesn't mean there isn't one.
Why?But what gives? I mean a dormant rose bush is just sitting there. How and why does it respond to someone cutting bits of it off? Well, I found some papers looking at dormancy in grapes, and they found that dormant buds are really quite busy, with many genes still being actively expressed. The also found that during natural dormancy breaking, the hormone auxin peaks in the buds a full two weeks before any visible bud swell. So, in late winter, when your plants look like they are just sitting there, they aren't. Genes are doing there thing, and hormones are churning, and when you take your pruners and lop something off, you change the patterns of gene expression, the flow of hormones, and can stimulate buds to break dormancy and start growing.
The bottom line: wait to pruneIt looks like the advice to avoid pruning too early in the season is good. By pruning too early you can cause them to begin growing to early, and result in more damage from late spring freezes.