Lavendar, dahlias and marigolds

off - my computer keyboard is a bit old and cannot type repeated letters.

As for the marigolds, if they are the common calendula, they will self seed, and if you want them next year, encourage them to do so. I have been trying to get rid of mine for some years now, but every year new plants spring up from seed and I let some be because they are very jolly and remind me of my childhood when every garden seemed to be full of them.
Dahlias are a bit hit and miss. You could dig up the tubers and overwinter them in sawdust or dry peat but there's no guarantee they won't deteriorate. If you live in a mild area, you can leave them in the ground and cover them with leaf mould. Nothing to lose really. Slugs are the enemy, and frost.
As for lavender, like Manuel, I know nothing. I'm still worrying about keeping DH's carefully nurtured, grown from seed, plants alive through the winter!

Lavender -if you cut into the old wood it won’t regenerate. So snip away but just the newer stuff.

Marigolds are an annual, but there should be lots of seed so will probably be loads next year.

Dahlias - yes I guess it depends where you live. We can leave ours in as well as gladiolus. Not sure why they haven’t flowered though? When did you plant them? Usually wait until the frost does for them before clearing them away —don’t know why though.

If your lavender is all over the lawn it’s days may be numbered (in your eyes) so won’t matter if you chop it back hard!

I bought marigolds this year. Put them in pots, still flowering . Have taken the dried seed heads, put them in a paper bag and will resprinkle in the soil in spring where they will grow, then take seeds again.
Lavender cut back but not into the hard wood. If you dont cut back the woody bit gets bigger and will mot flower.
Dahlias we used to dig up the corms or whatever the root bit is called and put in cool dry place over winter.

Gosh - lots of info. Thanks all.

Will the marigolds seed themselves or do I need to collect the seeds? Seems to be two schools of thought on this.

When should I cut back the lavender? - about now?

You can do either. If you are gathering seeds, wait until they are ripe though before you try and collect them. They'll come away quite easily when they are.

Lavender. Take several cuttings, usually roots quite well. Cut the plants back to green growth around now.
Marigolds collect the seeds and scatter them over the ground.
Dahlias, really don't know. Some people dig the corms up and replant them in the spring. Some people just cover them with a mulch and hope they survive the winter.