I get asked this frequently from people who want to propagate a row full of willow trees for a hedge, or setting up for basket making.
Most willows and some poplars are easy to propagate from cuttings. But even with these easy ones there are tricks that will increase your yields.
While you can take your cuttings any time, the best time is in late winter, when the snow is starting to melt. Each month after that will drop your success rate by about 10%
Take stems that were new growth last year. The ideal stem is about the diameter of your little finger to as thick as your thumb.
Alas, few are set up to plant them yet. Bundle them up, and bury them in snow on the north side of a building. Pile more snow on them, and tarp the pile. Two feet of snow and a tarp should give you a month.
Once leaves are coming out on other trees, your soil is warm enough to plant.
Dig up your bundles, are make your cuttings: If you are planting in soil you want your cuttings to be about a foot. If starting in #1 pots, about 6" If othersize pots, about the height of the pot. Start at the bottom of the stem, and cut at a 45 degree angle, about 1/2 to 1" below a bud. Make the top cut straight across about a half inch above a bud. The slant cut on the bottom makes planting easier and allows you to tell top from bottom. Discard the top when it's diameter is less than pencil thick.
Soak the cuttings in a bucket of cold water until you see white spots (adventitious roots) start to form on the stems. Don't soak until long roots appear. These 'water roots' don't adapt well to soil conditions, and are frequently broken off when planting. You have to dig a hole instead of just pushing them in.
Plant the cuttings either where they are going, or in pots. If planting where they are going, plant about 10% into pots to have ready replacements for ones that don't take. The cutting should only have 1 pair of buds above the ground for a tree form willow, 2-3 pairs for a shrub form willow. Tree form willows should be planted at about a 30 degree slant with the bud pair on the top side.
mulch heavily to retain soil moisture.
Water whenever the top inch of soil is dry.
For tree form willows, once new growth is about 3" long, remove any other shoots. Pick the one that is closest to vertical.
For shrub form willows prune all shoots when about 6" long. This will encourage branching.
Rooting compound doesn't particularly help.
Success rate is poor with some willows -- Bebb's is a good example. Most willows will have about 95% success rate.
This method also works will most poplar, but not with trembling aspen, swedish aspen.