Water newly transplanted shrubs regularly for the first few weeks if rain is undependable. Butterfly bushes do not require routine supplemental watering once they are established, especially if they are well mulched and planted in good soil that has lots of organic matter in it to hold water. In times of drought water them with an overhead sprinkler or a soaker hose system along with other shrubs and trees to soak the soil down to 8 or 10 inches. Mulch the soil over their roots to discourage evaporation of moisture from the soil.
Butterfly bushes do not need a lot of fertilizer. In good soil that is well mulched with organic material, this plant needs no fertilizer. If you have poor soil, in the spring sprinkle a handful of granular, all-purpose slow-acting granular fertilizer on the soil around the base of the shrub for the rain to soak in. This should be sufficient for the season.
A 2 to 3 inch layer of some organic material such as wood chips, shredded bark or chopped leaves spread on the soil over the roots of the butterfly bush out to the drip line of its branches will benefit it greatly all year long. This mulch discourages weeds, helps keep the soil moist, and, as it breaks down, feeds the soil to keep it alive and fertile. In the North it is advisable to mulch the roots of the butterfly bush over the winter. Spread a layer of chopped leaves, or some evergreen boughs if there is not enough snow cover, to insulate the soil from extreme temperature fluctuations that may disturb shrub roots.
Click here for more information on Using Mulch.
Butterfly bushes look their best when their faded blossoms are clipped off. This encourages continuous bloom. Also annual pruning is very important for butterfly bushes:
In The South: In the South stems from the previous year become woody and brittle if they are not pruned at the end of the season. Cut them back hard, down to within 4 inches of their base. Left unpruned, these bushes produce smaller flowers and are short-lived.
In The North: In the North, winter frost usually catches the current season's growth, causing the shrub to die back to the ground. In either the fall or early spring prune all remaining partially frost-damaged branches back to the ground to encourage vigorous growth and flowering for the new season.
An exception to this pruning schedule is the Fountain butterfly bush, whose stems are cold hardy. They survive winter and bear flowers on the existing wood from the previous growing season, so do not prune them until just after they bloom. Then cut back the stems just enough to shape the shrub and control its growth.
For more information see file on Choosing Pruning Tools.
Butterfly bushes do not require routine staking or support. Occasionally arching, long lower stems will crowd nearby plantings. In that case either prune them off altogether or set 3 or 4 stakes equidistant into the soil a foot or so out from the base of the shrub. Circle garden twine around the stakes at whatever height is necessary to raise and support the drooping lower stems.