During the winters of 1986 and 1987 and 1987 to 1988 I overwintered in pots, a number of apple, pear, plum, apricot, and sweet and tart cherry trees. The decision to grow things in pots was based on these considerations: First, a black plastic pot provides a warm environment for growing new roots on bare-root nursery stock. Second, it eliminates for the time being, the need to dig lots of holes in the yard when it may not be convenient.
If you are overwintering fruit trees indoors, the ideal environment is one of total darkness and temperatures around 32 – 35 degrees Fahrenheit. If the temperatures get above 40 degrees Fahrenheit for any length of time, extremely hardy rootstocks and cultivars will begin to break dormancy and they may do this as early as January. Under the best conditions, it will be difficult to keep even the hardiest material dormant much past March.
I overwinter a few “Wien” and ‘Renetka” crabapple rootstocks in 1987 to 88 in Bill Reynold’s greenhouse basement where temperatures stayed around 37 – 39 degrees Fahrenheit. They and two ”Pioneer #3” pear trees began to break dormancy in early January. I dealt with the problem by buring one of the “Pioneer #3’s” and the rootstocks in a snowbank after enclosing them in a large plastic bag. When they came out of the snow bank in late April, they began to grow promptly although a few of the furthest out buds had been killed by sub-freezing temperatures in the snow bank. The coldest air temperature above the snow was -11 degrees Fahrenheit.
The rest of the trees stayed indoors in total darkness until May 6th, by which time the basement temperatures have risen into the forties or possibly higher. I found etiolated shoots ( no green chlorophyll) growing from all my trees, and these shoots were damaged by direct sunlight when the plants were transferred outside.
By contrast, I stored in a moose-proof area in the yard, protected from the wind but open to the sky, some other apple, pear, and plum trees. These I mulched with 4 to 5 inches of leaves and grass clippings in late October, all around the pots. November and December were very snowy. The pots were covered at times by 30 inches of snow. When the snow melted and contracted, it broke off one or two of the lowest branches on two trees, but they were only partially broken. I taped them back together, and the repairs were successful. By May 6th, the “Viking” and “Mantet” apples and the :”Nova” and “Tyson” pear trees or breaking dormancy. There was no winter injury on the Viking and Tyson, and maybe an inch or less on the other two. There was no winterkill at all on the “Gravenstein,” “NY 652”, “NY 394”, “Tyrrustrup”, and “Merton Beauty” apples, all grafted onto Renetka in 1987.
On an 8-foot high, healthy “Toka” plum in a pot, there was winterkill of typically 20% of the 1987 growth. That tree was purchased in 1987. Another “Toka” grafted by the author in 1987 onto a prunus salicina mandshurica, suffered 50% winterkill of its two branches but grew vigorously after breaking dormancy period. Other trees acquired in 1987 and overwintered outdoors were “La Crescent” Plum and a “Waterville” pear. They suffered 20% and 40% losses, respectively of the 1987 growth but recovered well. Here are some general recommendations for overwintering trees in pots outdoors:
- Find a location with little direct sun and protected from wind.
- After the ground freezes, mulch the pots with 4 to 6 inches of leaves and grass.
- Protect your trees from moose.
- Avoid heavy snow loads on branches.
- Remove the mulch as soon as the birch trees begin to leaf out.
Overwintering trees in pots indoors is worthwhile only if they have very sparse root systems. Cherry trees are probably better overwintered inside, also. If you keep the trees inside, remember these points:
- Temperature should be kept below 40 degrees Fahrenheit and preferably below 35 degrees Fahrenheit
- Trees should be watered 1 – 2 times a month
- Monitor the buds, and put the tree outdoors as described above if it is impossible to hold back bud break indoors
- Trees should be put in a cool, lighted location in mid-april or else put outdoors if still dormant.