Q. We planted 'Zuni' and 'Hopi' crape myrtles this past spring, and they flowered their first year in the ground. How should they be pruned, and what is the best time to prune them? Also, how should we protect them through the winter? Those varieties are reported to grow in the Pittsburgh area, but we were told to cover them through the winter.

A. Crape myrtles (Lagerstroemia spp.) are not considered reliably hardy for us in USDA Zones 5 and 6. There are two primary species of crape myrtles, Lagerstroemia indica and fauriei. L. indica is reliably hardy from Zones 7 to 9, while L. fauriei is supposed to be hardy to Zone 6. 'Hopi' and 'Zuni' are hybrids between the two species and are reported to be hardier in northern climates. If you live in an urban area like Pittsburgh, your chances of success are better than if you live in the suburbs or a more rural area. Due to the concentration of pavement and buildings, urban areas tend to absorb and hold more heat than less built-up areas. I can think of several crape myrtles growing well in Swissvale and the city.

The way you plant and care for marginally hardy plants has a big influence on how well they survive. Be sure they are planted in well-drained soil; standing water around their roots through the winter is a recipe for failure. Avoid planting them against south-facing walls. On sunny days and during warm spells in winter, south-facing walls absorb and then radiate heat that can cause nearby plants to break winter dormancy. Buds that start to break, and any new growth will be killed outright when temperatures fall back to a normal range.

Avoid pruning or fertilizing marginally hardy plants after early July because these activities push new growth that may not harden off before winter cold hits. Older and more established crape myrtles will fare better than young ones when we get into severe cold snaps.

As for covering them through the winter, wait until the ground freezes, then mulch with 3 or 4 inches of shredded bark, straw, shredded leaves or well-composted sawdust. The point is to keep the ground frozen, which protects young plants from heaving out of the ground during winter's freeze-and-thaw cycles. Keep the mulch several inches away from physical contact with tree trunks. Be sure to pull the mulch back once we are past danger of hard frost in the spring. If your crape myrtles are exposed to strong winter winds, you can erect a windbreak by stapling burlap or erosion control fabric onto wooden stakes and driving them into the ground on their windward side.

Because crape myrtles produce flower buds on the current year's growth ("bloom on new wood"), they are usually pruned in late winter or very early spring.

Crape myrtles can be trained as single-stemmed small trees or as multistemmed shrubs, depending on your preference. Once you have the basic framework established, crape myrtles require only light pruning to maintain that form. Growing them as small trees allows you to limb them up to reveal their lovely exfoliating bark.

Northern gardeners do not always get to enjoy the bark because crape myrtles can be killed back to the ground during severe cold snaps. If you want to train yours into small trees, select three to five well-spaced shoots that grow from the ground as the main trunks and remove the rest. Remove the side branches from your selected shoots about halfway up their length. In subsequent years you may need to remove additional shoots that sucker up from the base to retain the tree shape. You can also prune any excessively long shoots growing from the main trunks back into shape with the rest of the tree or remove them entirely at their point of origin.

If you prefer to grow your crape myrtles as multistemmed shrubs, you can cut them back hard -- within 6 inches of the ground -- every spring. You will never get to enjoy that exfoliating bark, but you can keep them small. You can also prune them more moderately to allow them to grow into larger shrubs. Prune out weak, spindly twigs to open the interior of the plants to more sun and better air circulation. You can also shorten long branches back into shape with the rest of the plant.