What is the best way to overwinter the 3 blueberry plants I have growing in containers? Last year I kept them in the garage under grow lights, and only 1 bush (out of 3) survived after I moved them outside in the spring. The year before, I left them tucked next to the south side wall of the house, and none of them survived. Any ideas?
Over-winter in an unheated garage - from time to time, toss enough snow on the soil to keep the root mass from drying completely. Alternately, bury the container in the garden or against the north side of a building & mulch. They'll do fine as far as chill is concerned, and as long as you've attended to their other cultural wants.
With due deference to Al, I must state here that I grow more than 30 blueberry plants in containers, ranging from seedlings to substantially sized plants in fruit production in zone 5. My plants remain outdoors without special treatment, burying in the ground or sheltering. I have never lost one due to overwintering this way. Small plants in small containers (< 1gal) do remain vulnerable to too much drying. As long as you are not growing southern varieties, should should be Ok without any necessary special intervention. However, it does not hurt to mulch or bury the pot, provided you use an acid mulch and provided that the roots do not end up being submerged in water too long during the spring thaw.
My blueberries get an inch or more of peat added to the pot every spring and occasionally in the fall.
Thanks for the suggestions. I plan to place the pots in larger pots, lined and topped with shredded leaves, along the south side of the house. This area is on a slope which should provide good drainage in the spring. And since they'll be outside all winter, there will be no drastic change in light intensity as there was last year. I hope this works.
You'd be better served to site the plants on the north side of the house out of direct sun. Photo-intensity per se is not an issue, but the solar gain (warming of soil and the plant) as an indirect influence can facilitate sap rise and subsequent freezing.
That you don't actually kill a plant by exposing it to extremes in cold is not a clear signal that these cold extremes are not causing damage to the plant, or that it should be taken that all is a-ok because the plant is not dead. It should be interesting to note that in situ plants, even in zone 4, rarely see 6" soil temperatures at or below 25*.
There is no way to tell how hardy any individual plant is w/o killing it. Cold-hardiness is a genetic trait & is passed through the generations of vegetatively propagated plant material. Plants that are able to grow and fruit well nearer the equator may have nearly no tolerance to freezing temperatures, while trees originating from a more northerly provenance that grow and fruit well will probably have a considerable ability to withstand cold.
When it comes to how much cold roots can tolerate - all roots are not created equal. The finest, most succulent roots succumb to freezing first, then the medium diameter, more lignified roots; finally, when temps get too cold, the more perennial, well-lignified hold-outs finally succumb, causing the death of the plant. That you cannot see this damage isn't enough to suppose it's not occurring - it is. The effect of this incremental root death is often evident in late leaf-out in spring. The plant will expend a fair amount of energy developing rootage to support the canopy before the plant leafs out. The result is a plant with lower energy reserves to push the spring flush of growth; or a loss of potential increase in biomass. Many plants, after exposure to cold extremes, are pretty much the same as large cuttings, come spring - because of the chill injury to roots.
I'm guessing the damage to the finest rootage of the more hardy varieties probably starts around 25-27*. I'd like to say that there are few BBs that will tolerate ACTUAL root temps much below 15* w/o dying or considerable damage, but I'd probably get conflicting anecdotes, so I won't. You CAN see though, that at these extreme low temperatures, there is going to be considerable freeze damage to secondary and tertiary roots, and why it's much better to keep the plants roots at actual soil temps not much below 25*.