Q. I planted three blueberry bushes in large containers in my back yard. I know it's still the height of summer, but I want to have a plan in place to keep them alive over the winter. The way I see it, I can do one of three things:
1) Move the containers to my uninsulated, South-facing, enclosed back porch (which gets good sun, but still gets very cold at night).
2) Build a small "greenhouse" for them out of corrugated plastic. Or
3) Mulch the tops of the containers and not give them another thought until spring.
Eek! Do I have a chance? I've never tried to keep container plants through the deep freeze!
--- Paul in Chicago
A. Well, I think that Paul's chances are very good, because he's thinking about it now instead of waiting until the poor plants have frozen solid in January. Growing small fruits in containers has become very popular, but many people aren't thinking about how to protect the plants over the winter in cold climes.
"Can't they just come inside, like houseplants?"
Many can. Add some artificial light and this works great with plants like begonias, dahlias, rosemary and peppers, which only need protection from freezing cold. (And rosemary only needs it in [roughly] climates that either are or feel like they're North of the Carolinas in the winter.) But blueberries, like many Northern fruits, also have what's called a 'chilling requirement'—a certain number of hours over winter that they must be exposed to temps that drop down to at least 40 degrees F.
Not below freezing; that's a common misconception—and why they call it "chilling hours" instead of 'freezing hours". Now it will drop below freezing during the winter in many places (like my PA garden), but all you need are X number of hours (each variety has a different requirement) below 40 degrees (and in some 'low chill fruit' cases a mere 45 degrees F.) to insure good fruit production on plants like apples, peaches, raspberries and blueberries.
Now don't get me wrong: blueberries are basically incredibly cold hardy. The lowbush varieties that grow wild in the North are hardy down to USDA growing Zone 3, where winter temps can drop down to below 30 below. And even the taller highbush types are hardy to zone 4, where winters are still very severe. BUT as we have explained in the past, these zonal declarations are for plants with their roots in the ground, not up in the air in a container. And none of our Chicago listener's ideas seem to change that fact. In all three cases, the roots of his plants would still be 'out in the open'. Let's review:
Idea number one was a South facing shed with good sun but no heat. That's a double no-no; the bright winter sun shining into a closed area might bring the plants out of dormancy on a really warm day; and then they'd be even more vulnerable to freezing to death on the next really cold night. This rules out the plastic greenhouse too; same thing would happen there.
The third idea was to "mulch" the plants heavily and hope for the best. This idea might work from, say Philadelphia South—but Chicago can get really cold over the winter, and their winds are legendary. That's important; strong winds probably cause more plant loss in the winter than direct cold because they're not only wicking the moisture out of plants, they're wicking that moisture out of plants that are dormant and can't replace it. And since "mulch" technically means covering the surface of the soil, these plants are still sitting out in that windy openness.
But the basic concept here has merit. If he wants to try this route, I would suggest moving the containers as close to the house as possible (the classic 'sheltered area'), covering them completely with shredded leaves and surrounding them with burlap wrapped against some fencing that keeps it a few inches away from the plants.
…Not just draped over top of the shredded leaves and plants. Wrapping plants IN burlap is a common rookie mistake. Just think about it: Burlap that's touching plants will get wet, freeze and crush them pretty early into a real winter. Burlap is best used as a true windbreak; positioned close to, but not touching the plants.
Now ideally (and especially if we're talking about big trees, like potted apples or figs), I'd suggest turning the containers on their sides, rolling them up against a non-South facing side of the house, completely burying them in well-shredded leaves and then digging them out and standing them back up early in the Spring. (Non-South-facing for the same reason as the sunny unheated room; the South facing side of a building could warm up enough to bring the plants out of dormancy during a really sunny winter week.)
Now: Our listener says that his containers are 'large'; what if he can't move them to a sheltered area? Well, he also says that one of his options is to move them onto a porch; so how big could they be?
Anyway 'big is good' in this instance. In general, the bigger the container, the better the chance of winter survival for the plants living in them. So if his blueberries are growing in something huge, like half whiskey barrels, he might be able to cage them, fill the cages with shredded leaves, wrap burlap around the cages and leave them out in the yard. Although turning them sideways, rolling them up against the side of the house and burying them is a safer bet. (At least for the plants; the pots need to be made of something that won't crack apart when it freezes.)
But the gold standard, of course, will always be to move the plants into the ground for the winter. If you have potted plants that might not survive the winter aboveground, but that are rated safely for your USDA Zone, planting them—even temporarily—virtually insures success.
After the plants have gone completely dormant, dig good sized holes in your yard (or next door [with permission!] or at a friend's house, or…you get the idea) and drop the plants into the holes at the same soil line level as they were in the pots. Pick an area that drains well; not a low spot where water pools up. Heck, you can even leave the plants in their pots if the pots are made of something that won't break.
Then fill in the holes in with the same soil you removed, water the plants well once, and then just leave them be until Spring, when you can 'lift' them and put them back into their pots. (Or if they're still in their pots, back in place. This plan should work perfectly for blueberries; they're tough plants that transplant very well.