Amending Soil - Existing Beds

Good day all! I've just started, as we recently moved. Never had a garden before. Apparently the people who lived here a LONG time ago did garden a little, but the beds fell out of use, etc. I've already removed all the weeds, and am left with a few surviving perennials, shrubs, roses, etc. My question is this: the soil does need to be addressed, but how does one amend soil when the plants are already in it? I've seen plenty of articles on how to start, but not what to do when a garden was (at one time, anyway) established. I'm concerned that adding the oft-recommended 1-inch compost dressing will merely smother what's coming up, and that working the soil with a hoe or spade will kill off any potential re-seeders. Please advise.
Thanks!

You may want to get a soil analysis first to check if any admendments are needed. If you layer on some mulch plants will just push their way thru it, so you don't have to worry about that.
I always wait a full growing season to see what comes up and then make changes to the plants.

I agree with flowerjen - try getting a soil test. most universities and colleges with a science department will be able to do this - colleges with an agriculture or botany program may even do it for free. Either way, you get a scientific report explaining the exact pH, sand/silt/clay ratio, and other stats. Most reports will have a 'layman' explanation of what the soil needs along with it.

However, in my opinion, the easiest and most failsafe way to make the soil better is to simply add organic material, as flowerjen said. Anything you throw on top of the soil will eventually make its way through to the soil underneath.

If the underlying soil is very sandy or very clay, however, it will affect the amount of nutrients it can hold. In these cases, organic material must be physically mixed into it.

For the bigger shrubs, you won't really do any damage by turning the soil as long as you stay a few feet away from the base of it. Although it was marketed to seniors, the "Garden Claw" tool is one of my favourites for mixing in new organic material. It seems to get under and 'lift' the roots of shrubs and perennials instead of cutting and tearing like a shovel would.

Many types of perennials benefit from being dug and divided - so figure out what they are and do a little research on best time to dig them up and any special treatments they might need (for example, siberian irises need to be dug up, separated, rotted sections of root cut out, and given a mild antiseptic bath every few years to keep healthy.)

If you're ever to change the layout of the garden, an even simpler way to get good soil is to remove all the plants at once, add a foot of good triple-mix onto the existing beds(roto-till the first few inches of t-mix into the old stuff), and start over! However, this is not always practical.

I agree on the soil test-- here's a good link: http://www.oldhouseweb.com/gardening/Detailed/780.shtml

Also, you can never go wrong with good compost with manure in it. Just dig around the outside of the plant-- leaving a wide enough margin so you're not going to hurt the roots and work it into the soil. Also, add a general organic fertilizer like Milorganite, which can be used anywhere and is pretty inexpensive at most garden stores (not the big boxes...they don't typically carry it.).

And, remember to water deeply every 3-4 day when there isn't rain. Don't water a little very day or so or the plants won't establish deep roots.

Thank you very much for the information. It would appear that someone had a beautiful garden here many moons ago, b/c there are some interesting things coming up, but it also appears they didn't do much amendment to the soil. :-|
I'm going to try to claw some compost around things and then just top with mulch after I get the test (I bought a kit). Thanks everyone~!