Winter maintenance

I am a new gardener. I wonder what I need to do to my perennial garden to prepare it for next season. I live in zone 7, SW Virginia.
My garden consists of 2 Butterfly bushes, 2 Coneflower's, a lavender plant, 5 red hot poker plants, 5 mum's, 1 aster, 3 Canna bulb plants, and a Knockout rose bush.
Do I need to cut them all down to ground level and mulch them well, or just leave them alone?
Any help would be greatly appreciated.

Phil

Here is how I would handle these in zone 9b- minor frost, not hard freeze.

2 Butterfly bushes- (Buddleja davidii, hybrids and related plants) There are several varieties that grow to different sizes. If it is one that grows a bit too large I would cut it harder, but in general:
Remove 2-4 of the oldest stems as close to the ground as practical. Cut off the others just below the faded flowers. Remove any broken branches to a point below the break, just above a pair of buds. On the spring, make sure that these upper buds are sprouting, or cut the branch lower, to some buds that are growing. Alternate method: Cut the whole plant down as low as you want, and treat it more like a perennial.

2 Coneflower's- Remove pretty much everything to perhaps a 6" mound of stubs. I leave these to show where the plant is. A small stake would work as well.

a lavender plant- Trim off old flowers, barely getting into the leaves. Around here, Lavender grows year round. Cutting too hard can encourage too much new, tender growth that might get nipped by the frost. In late winter (February) give it a harder pruning, back to about 25% of the mature size (4' tall plants would be cut to 1' high, 2' plants would be cut back to 6" high). Remove dead branches.

5 red hot poker plants- To me, these are almost a wild plant, let them fend for themselves. I see a lot of them at the ends of a driveway leading to a ranch. Nobody does anything for them to see them through the winter. You should remove old flowers. In late winter you could cut back the whole thing to perhaps 6"-12" high. This gets rid of all the old leaves before there is danger of damaging new growth, but after the chance of frost that could kill the new growth.

5 mum's- Clean up of old flowers, cut back, leaving enough sticks to be able to know where the plant is. Like Coneflowers, a stake would work, too.

1 aster- Might depend on species. Some do not look too bad through the winter, so let them be, giving them a good pruning in late winter. It is just fine to cut the whole plant back to a 6" mound now or in spring.

3 Cannas- I just let mine be. By the end of winter they are looking pretty shabby, so I cut off all the stems in late winter. Any earlier and they would try to grow in the mild spells, then get nipped by frost. I wonder if in your zone they would stay dormant, and be best covered in mulch.

Knockout rose bush- Do the fall pruning with an eye to how the plant will respond in the spring. You want to be able to encourage symmetrical growth toward the outside of the bush.
You will likely be removing leaves and flowers, perhaps even flower buds. I generally cut back most roses to about 50% of their mature size, thinning out any branches that are too old, broken, or had not produced flowers. I try to leave at least 5 branches equally spaced around the plant (more on an older plant), and most of these may have 2-3 branches growing off them. I shorten these to an outside bud. Dig in around the base of the plant if you see any growth that could be coming from the rootstock. Remove these. I then do a dormant spray with the goal of reducing overwintering insects and diseases. You would tailor this to your local pests and diseases.
You can prune harder. Some roses do not really know about going dormant. Around here I prune most roses this way in November or early December, about the time frost would be expected, so the frost will keep them dormant until spring. Roses here do not need to be covered in mulch. You might need to, but someone who gardens in your zone would better be able to say if this is needed.

All plants:
Rake the soil around them, blending the old mulch with the soil. Fix up any berms, if the plant needs them. (young plants get a berm to hold water near the root zone, move the berm out seasonally to encourage the roots to spread.) Add a slow release fertilizer. If you want to use organic materials, this is a good time to apply blood meal, bone meal, greensand or whatever else your soil lacks. It won't really become available very much through the cold winter, but will be there in the early spring ready for the plant to use.
Apply new mulch. I often use compost that is still a bit chunky, or tree company chips. I just put this around the plant, not over them. I do not need to cover any of these to protect them from frost- they seem to do just fine in my mild zone- just enough frost to keep them dormant without freezing them.
In your colder zone you might need to cover them.