The process of training a shrub involves selectively pruning branches to maintain a good structural form and the overall health of the plant. The weeping blue willow (Salix caprea), sold as the cultivar Kilmarnock, typically grows grafted upon the trunk and root stock of another willow species. The pendant branches of the weeping blue willow cascade down the trunk from above a swollen or scarred area known as the graft union. Never prune below this union; if you do so, you completely remove the plant tissues of the weeping blue willow, and the root stock will sprout its own non-weeping shoots. Grow weeping blue willow in USDA hardiness zones 5 through 9a.
Look at the length of trunk on the weeping blue willow. Prune away all buds or sprouting leaves and stems that originate on the trunk below the graft union. Buds easily break off with your fingertips when small, but sprouted stems need removal with hand pruners. Make the cut flush with the trunk, avoiding damaging or tearing any bark on the trunk itself. Do this trunk cleaning in early spring and again in late summer.
Reduce the length of any weeping branches that touch the ground. Prune them back to a point 1/4 inch away from a bud or leaf that is 12 inches from the ground. Keep branch tips from reaching and sprawling across the soil since they will root and become new plants. Do this pruning in early spring and again in midsummer.
Prune away dead and broken branches any time of year. Make the pruning cut 1/4 to 1/2 inch above a lower branch junction, leaf or dormant bud if the plant is leafless.
Scan across the canopy of weeping branches on the weeping blue willow, looking for rubbing branches or any upright sprouts with tip ends pointing upward. Prune away the weaker or less attractive branch if two branches cross each other or physically rub bark and make a wound. Remove all vertical sprouts from the top of the canopy. Keep in mind sprouts that grow upright but more horizontally do not need removal, since as they lengthen, more horizontal twigs form that eventually droop.
Thin out and balance the remaining canopy of branches on the weeping blue willow. Overall, branches need to radiate outward in all directions from the graft union on the trunk. A uniform, balanced canopy withstands wind stress and maintains even weight load after heavy wet snowfall or ice accumulates on branches in winter. Remove tiny twigs or branches that do not add to the structure or beauty of the canopy. Remaining branches need good exposure to both direct sunshine and air circulation.
Schedule the first annual training pruning for late winter to early spring before buds open. This gives you unimpeded view of the plants branching structure. Follow up with light pruning to maintain the structure and form of the weeping blue willow monthly across the rest of spring and summer.
Avoid doing major pruning between midsummer and the first fall frost. The plant's response to grow new leaves and stems from the pruned area is problematic this time of year. Chances are the new growth won't mature fully in time to survive the initial cold temperatures and frosts in fall and will be killed back.