The question keeps coming up. Should I cut back the dead foliage on my perennials in fall or wait until spring? This is an important question and deserves a lot of research before I can answer it emphatically. Research done. The answer is . . . please yourself. I say please yourself because the benefits or drawbacks are more relevant to how you feel about your own garden.
Picture this: You have party at your house; it's 3 a.m. and you've just slammed the door on the last guest. Are you the type that washes all the dishes, tidies up, and then vacuums before going to bed, or do you simply collapse in the squalor? Of course, the answer is probably somewhere in between, depending on how the party went and whether you feel like holding another.
It's much the same in the garden. Whether you cut back the perennials or not largely depends on how you feel about the way things look, or whether it's the front garden or the back. Cut back or don't cut back? More than likely it won't make much of a difference. No one has ever come by my garden in summer and said, Ah, I see you didn't cut back your veronica last fall.
There are practical reasons for cleaning up immediately after a party and there are practical reasons to tidy up the spent foliage of perennials in the garden, but there will always be an opposing opinion, regardless.
In the garden, the pros and cons usually go like this: leaving all the stalks and seed heads on plants will provide food for birds during winter, meanwhile snow will collect and build up on the flowerbed, protecting the tender crowns of plants below. The mounded snow will also be aesthetically pleasing to the eye. On the other hand, insects and disease can remain with the foliage allowing them to be on site in spring ready to have another go at the plant.
Is the latter a concern? I'm not convinced. If you have plants that have obviously been afflicted with disease this year, then by all means remove and destroy the foliage, maybe the whole plant, but accept that many fungal and viral diseases are caused by organisms that winter over in the soil. Finding a needle in a haystack is a breeze compared to picking fungus spores from soil, and if you don't get every last one of the little devils, the problem will be back. The severity, however, is more apt to depend on weather conditions, rather than your diligence.
Remember the tar spot fungus that was plaguing maple trees the last year or so? It caused unsightly black spots on the leaves and we were warned to clean up every last leaf around the garden (I composted mine regardless). There's not much sign of tar spot this fall, but I'm sure it isn't because every leaf with a black spot on it was conscientiously removed from the province. More likely, it was a dry spring that disrupted the spread of spores. This is the cyclical nature of insects and diseases.
I'm afraid I still haven't answered the real question, so if it helps, here's what I do. On my roses, I cut back any extra long canes that will whip about in the wind, but leave pruning until next spring. I will also leave woody or evergreen perennials alone, but I might, if I'm in the mood, remove the mushy dead leaves of herbaceous plants like day lilies or hostas as these can provide hiding places for slugs to hide out. Unlike woody perennials which sprout from their stems, these plants sprout anew from their roots. I will wait, however, until frost has finished them off. Ornamental grasses sprout from their roots, too, but I wouldn't dream of cutting them back until spring. They are a highlight of my winter garden.
If you're still not sure about which perennials to cut back, take a clue from Mother Nature. After the party is over, she throws a blanket of leaves over the whole mess and doesn't worry about a thing. Don't you worry so much, either.