Here's a review of the best way to keep your annual geraniums and other plants going until next summer.
To begin with, remember the things that made them great this summer.
Annuals need a great deal of both sunlight and heat combined with adequate amounts of water and fertilizer to bloom heavily and continue growing.
But winter will reduce the amount of light available and it will surely reduce the heat available so a few changes in watering and feeding are required.
Begonia 'Dragon Wings Red'
The most obvious, in fact I almost hesitate to mention it, is to put
your overwintering plants in a frost free place. There's nothing like a
prolonged frost to put your precious annuals into a permanent dormancy.
Once we've got that rather specious bit of advice out of the way, let's look at some practical advice.
The first is to put your annuals in the brightest and sunniest spot you
can find. Remember that light levels go down in the winter and the more
you can give them, the happier they will be.
I know your grandmother used to overwinter hers in the dark basement, but were you ever around in the spring when she brought the pure white, stretched out, fungus-ridden survivors back into the light of day? So, yes you can overwinter a geranium without sunlight but you just may not want to when you see what the outcome is.
Even with a greenhouse, we would not be able to stop the geraniums from stretching in December and January light levels. The more light - the better. This bit of advice holds true for every annual you are going to overwinter - from geraniums to fuschia to leonotis.
Lower light levels also mean you can lower the temperature to slow down
growth rates. Most annuals are quite content with 62 F and will even
live at 45 F for December and January.
Lower temperatures help the plant slow down its growth rate and when combined with lower light levels, the plant will not stretch as drastically.
So, a back bedroom where the heat vents are shut off is fine for overwintering annuals. Because we are slowing down growth rates, we also want to lower the amount of fertilizer we are using. Cut back your weekly outdoor feeding to a once-a-month feeding during the low light winter months.
I also cut back the watering on my overwintered annuals. Where I would water fuschia baskets almost daily during the summer season, during the winter they are lucky to get a weekly watering. I don't want them bone dry but neither do I want them to be soaking wet all the time.
I try to let them dry down between waterings but I confess when I do water, I soak them as thoroughly as I can.
The real trick comes next March when the light levels outside start to
increase and the plants respond with new growth. That's when you have
to chop the thin, spindly growth back quite heavily on your overwintered
plants to encourage them to form thicker and more abundant growth.
Remember that on annual plants, the more new growth you create by hard pruning, the more flowers you get. Each new shoot will produce flowers much more heavily than will an older, spindly shoot.
As a next to last note, let me point out that yes, a few folks did indeed overwinter their bareroot geraniums by hanging them up in their basements over the winter.
What proponents of this garden lore don't mention is that the basements usually had dirt floors that kept the humidity high and a distinct lack of heat because they did not have central heating in those "olde" days.
Cool and moist roots would survive this kind of treatment. What they won't survive is being left alone in a modern basement where the air is dry and the heat is on - they simply dry out and die. Yes, you can keep them in a bushel basket if you water the roots every so often to stop them from drying out. Having said that, this is also a recipe for fungus problems and rotting. So, you can try all these things but your success is a function of how old and outmoded your basement is.
Datura 'Double Cream'
And as a last note, I've overwintered almost every annual plant that I
ever grew in my garden. From abelmoschus to zinnia, annuals can be
overwintered if you are dedicated enough and/or crazy enough to try.
In the case of plants like zinnia, it is hardy worth the effort - their seed is much easier to store than trying to keep them alive.
In the case of that magnificent fuschia or geranium, they are well worth the little extra trouble because they do reward you with the odd winter bloom and they do produce wonderful early blooms next year.