I have a dwarf white pine that suffered a fair bit of winter burn this last hard winter. My horticulture courses said you could trim white pines for hedges. I would just like to trim once for shape.
What will taking a pair of hedge shears do the eventual look and shape of a white pine?
If I trim it once do I have to keep trimming it?
If the burned branches are completely dead, your first step is to cut out the dead parts. Keep in mind that white pines do not regrow from old wood, so even if the base of a dead branch/branchlet is alive, you should cut it back to the next growing part (trunk, branch).
The next step is the shearing. Be aware of where viable growth buds are located and do not cut back behind these, even if there are needles. There are usually a group of growth buds at the base of the current seasons growth. Cut the new growth halfway back if there are buds halfway up the new stems (there should be). Do not cut into the previous seasons growth, unless there is a very long, unbalanced shoot.
If this is a specimen plant, I would do it with hand pruners rather than shears. This takes longer, but does a better, more accurate job and does not damage the needles. Also, if this is a one-time shaping, shearing won't be quite as attractive eventually, as the growth will all thicken at the same point, looking unnatural.
With hand pruners, you can make the cuts at varying points for a more natural look initially and in the long run. Now if you prefer the look of a formally sheared tree, you will want to shear it every year, to keep the tree looking good.
Shearing should be done as soon as the newest growth is finished growing, before it hardens. Natural pruning is often done in two stages: Cutting out poorly placed candles in spring before they mature, and then after they are hardened, cutting back the remaining growth to the desired look, again, keeping in mind where next seasons growth is coming from.
Below are pictures of sheared versus naturally pruned white pines:
