I don't have any personal experience with cover crops, but have accumulated some anecdotal knowledge here and there.

>>I'm wondering if experienced gardeners really do this.

Seems to be more commonly practiced by more serious, larger scale vegetable growers and an increasing number of environmentally savvy commercial farmers. It really is a good way to get appreciable amounts of nitrogen into the soil without overfertilizing the soil leading to other problems.

>>If so, how late in the year can you do this effectively -- my flowers are still robust.

October - perhaps even late October. I don't know, it might even be possible to plant certain cover crops in November.

>>Can you just sow seed directly on the ground?

Yes, that's how it's done.

>>And is there anything you like to plant that doesn't require tilling or spading next spring when you convert it back to flowers?

Tilling it under in the spring a few weeks before you plant your summer stuff is really the key to making the whole thing work. Being nitrogen fixing, colonizing plants, cover crops tend to be very competitive and prolific if they are allowed to go to seed. If you plant an annual cover crop and don't till it in, it will go to seed, then you have a weed problem. If you plant a perennial cover crop (something I haven't really heard of, but could be possible) and don't rototill it in (perhaps even if you do), then you have a weed problem - it will both persist and compete with your other plants, as well as go to seed.

I'll probably restrict the use of cover crops to my vegetable garden, for the time being (starting this fall perhaps). Maybe future discoveries will result in wider applications for cover crops that will be more useful to home gardeners.

I'm thinking about doing the cover crop thing also, because my garden is on a slope, and I get a lot of water flow in the winter. So I need something to hold the soil in place. But I don't want a crop that spreads by underground stolons or whatever. I gather that annual rye would be a good choice, and that certain fescues also will not spread vegetatively. I guess you could plant just about anything and then kill it with Roundup next spring, without having to till, but I am trying to avoid that. Is annual rye a good choice? If so, does anyone know where I can buy annual rye in the Portland area? The big box stores only seem to have mixtures.

In and around Portland, latest date I'd suggest is mid-October. But early October would be better.

The problem is that the longer you wait, the cooler the soil, the slower the germination, the more sparse the stand.

The most practical use for cover crops is to both protect and enrich the soil in fallow areas like vegetable gardens during the winter months. In addition to warding off compaction and erosion of the bare soil in winter weather, cover crops rightly earn their label as "green manures", as incorporating them back into the soil by tilling them in in early spring adds much needed organic matter to the soil as well as an assortment of basic plant nutrients.

They would seem to be much less effective if a) you killed them off with RoundUp or some other similar herbicide, or b)attempted to use them in any more permanent planting situation, like a anything other than an annual flower garden. The act of either tilling or using a herbicide to kill them off would inevitably damage whatever permanent plantings might be located in that area.

For perennial flower gardens (or any other planted area for that matter) applying a good layer of mulch - like shredded fall leaves or whatever you have on hand/purchase - can have virtually the same effect as planting a cover crop but without the hassle of removal. A mulch layer of 2-3 inches will also deflect soil compaction and erosion, reduce weed development, insulate soil temperatures and, in time, add to soil enrichment.

Annual rye is a commonly used winter cover crop that can be seeded right up to the first hard frost. Look for it in garden centers (labeled as a cover crop, not a grass seed) or at feed or farm supply stores.

I use woodchips in the ornamental garden. That, combined with groundcovers, does the job. I even give the groundcovers a top dressing of woodchips. Back on the farm north of Spokane we used annual rye in the veggie garden.