This being my first full year of having a garden I'm in a quandary as to what I should cut down and what should be left along.
Is there a rule of thumb for this???
Pretty much as many thumbs as there are gardeners... :-)
What's in your garden? Do you have annuals? Perennials? Vegetables? We all tend to do things a little differently depending on what we grow; some of us cut things down and do a complete clean-up in the fall; others wait until spring letting the foliage act as a kind of mulch over the winter.
Right now, a lot of perennials are still green and the annuals are getting their second wind so I'm not cutting anything back yet and will probably wait until the first frost. But when the time comes, I'll cut back the nepetas, Veronicas, daylilies, Oriental & Asiatic lilies, ostrich ferns, baloon flowers, hostas, bleeding hearts, peonies, baby's breath, trollius, tall garden phlox, Siberian iris, tall bearded iris, salvias, Oriental poppies... I leave sedums standing, and someone in a thread a few down on "Spent Perennials" recommended not cutting Russian Sage until spring when new growth appears. Annuals get pulled and thrown into the compost.
I've always liked cleaning up in the fall so I'm not trampling around in wet gardens cleaning up mush around new growth in the spring. I also like to start piling up the compostable materials to get that starting to "do whatever it does" over the long winters.
Since you're posting on the Perennials forum, I'm guessing this question applies mostly to perennials :-) You might want to invest in or at least check at your library for a copy of The Well-Tended Perennial Garden by Tracy DiSabto-Aust. This is an invaluable resource for the perennial gardener as it outlines growing conditions and care requirements for hundreds of species of popular perennials, including how and when to divide and what, how and when to clean up or cut back. And it is pretty much directed at the middle America gardener, although much of the information is applicable throughout the country.
Last fall, I left the back garden alone until spring and noticed more than the usual number of birds feasting on the seeds. This fall I will do a modified clean up and leave plenty for the birds.
What about mildewed plants? Is it best not to put those in the compost?
I toss them out, along with black eyed susan foliage and peony foliage. The BES always get some sort of fungus, or leaf spot, and while it doesn't harm the blooms, I don't want it spreading...it does anyways. :(
So my rule of thumb is to toss any foliage that looks, or is, diseased. But I don't get high temps in my compost...maybe that would kill the spores, not sure. To be on the safe side, out the suspects go!