I live in hardiness zone 6b. So plants need to be hardy to survive the winter. That’s even more the case if they’re planted in containers rather than in a garden bed. The moisture in containers freezes – freezing isn’t the problem – thawing is. Our weather is variable, so unless we get a substantial amount of snow which buries the pots, they’re susceptible to heaving which kills the plants.
I’ve had amazing success with quite a range of perennials: hosta (can’t kill it), chives (very hardy), pansies (will reseed themselves – so you have to learn to recognize a pansy seedling and not pull it out as a weed), forget-me-nots (don’t come back in the pots, but I shake the seeds from the plants over my garden beds and come spring I transplant patches to containers on the deck), Siberian iris, sedum, vinca (sometimes), peony, dusty miller. Ajuga sometimes returns, but not always – I usually treat some of these ground cover plants as annuals and plant anew each spring.
Lots of shrubs will survive in containers surprisingly well – I’ve had good luck with dogwood (including the variegated variety), rhododendron, spirea (these will survive a couple of seasons before they start to poop out), ninebark, and clematis. I’ve tried hydrangea, black cherry, current, hibiscus, but they’re only happy for the one season – they don’t make it through the winter in zone 9.
I’ve got a happy maple tree, a yellow birch, a ninebark, a dogwood all of which began as volunteers! I’ve tried lupins, bachelor buttons, poppies, hollyhocks, euonymus – all unsuccessfully. I’ve never been able to get them to return. I’ve enjoyed them, however, the season I planted them so it’s a matter of whether you’re willing to treat these perennials/biennials as annuals and replant them each year. BTW, I gave up on lupins because in a container the ants farm millions of aphids on them!
Come middle/end of May I go for what I call an “instant garden” – that means, adding annuals to the returning perennials as early in the season as makes sense (here in Nova Scotia our rule of thumb is no annuals before the first full moon in June – but my back deck is sheltered so I push the season a bit and often plant last week of May if I can). When I plant is also dependent on when the garden centres open and the variety of plants they’ve got available.
As you can see I plant more than one kind of plant in a container most of the time – it’s tricky getting the right balance of plants to pot size – in the spring when I’m planting, the seedlings are small and I have a tendency to overplant. It’s hard to remember that in a month or so, the pots will be full. Too many plants means they’re competing for food and moisture so don’t be afraid to leave space between plants when you’re planting them.If you live in a warmer climate zone – give it a try – the worst that can happen is whatever perennial/shrub/tree you’ve planted won’t survive the winter. Remember you’ve had one happy season from it!