I was checking over my geraniums this week. I’ve got a few I’m overwintering for myself, plus a few for a couple of clients, and then there’s another client who has a whole balcony full of them and needs them all lifting in autumn, overwintering and then setting back out in spring.
You’ll hear lots of different methods of overwintering geraniums (I should say, more correctly, pelargoniums), but here’s what I do. It’s pretty simple.
You take the plants out of the pots they’ve been in all summer, and pot up in a plastic pot which just fits the roots (i.e. you don’t have too much spare soil left over). General-purpose compost is fine – anything too loamy and they get too damp. Then I take the secateurs to them: any leggy stems are cut back to a bud or leaf joint about 4-5″ (10cm or so) above soil level. It seems drastic, but it keeps the plants compact as they’ll grow again from these points next year instead of starting a foot or two up in the air.
I give them one, very light watering, taking care not to wet leaves or stems, mostly just to settle them in to the pots. And then I switch on the greenhouse heater with the thermostat set to a few degrees above zero, and leave them to it.
I water them maybe twice the whole winter long. There are two secrets to overwintering pelargoniums successfully: first, they need to be very nearly bone dry, so once you’ve watered them when you transplant them, that’s pretty much it until February.
Second, you need to check them over at least once a week and remove any dead or dying leaves and stems. Botrytis, or downy mildew, or whatever that fluffy mould is that grows on dead geranium leaves is murder for overwintering plants and will spread like wildfire. You need to remove the leaves regularly to keep it in check, and take them out of the greenhouse too so the spores aren’t hanging around. If you do it regularly, you’ll find the fresh leaves will stay fresh and you’ll have greenery all winter. Harden them off carefully in early May, when you’re sure frosts are past (probably a bit later further north) and you can keep them going for years.