· Brent Wilson's Answer
· On the newest USDA Plant Hardiness Zone Map, Long Island is in zone 7. However, zone 7 is a little different in Long Island than say in zone 7 of Georgia. Long Island does not get the intense heat Georgia does. Also, while winter lows go down to the same numbers (around 0 Fahrenheit), Long Island can have extended periods with single digit numbers while in a more southern zone 7 these low temperatures will occur on a few nights during winter. So, if you're having problems with die-back on your crape myrtle, it's probably due to the longer duration of single digit temperatures. Unfortunately, there isn't much you can do to protect the tops of crape myrtle trees from severe cold. You can always apply mulch to protect the roots. That being said, there are types of crape myrtle that are more cold hardy.
There are several species of crape myrtle (Lagerstroemia) and some have better tolerance to cold than others. In general, Lagerstroemia 'indica' species are hardy to USDA Hardiness Zones 7 to 9 but are often killed to the ground in severe winters in Zone 7. But even if a severe winter kills most of the top growth on your crape myrtle, it is capable of growing back in a relatively short period of time. Since they bloom on the new growth, the injured plants are able to produce flowers as well. In the coldest portions of their range Lagerstroemia 'indica' probably won’t be able to develop a main trunk and the beautiful exfoliating bark that they are known for developing further south.
On the other hand, I've heard that Lagerstroemia 'fauriei' - the ones with the Indian names, such as 'Natchez', 'Muskogee', 'Miami', 'Souix', and 'Tonto', are reliably hardy as far north as USDA Hardiness Zone 6. The hybrids that have Lagerstroemia fauriei as a parent are hardier than Lagerstroemia indica cultivars, and develop into large specimens even in the colder parts of Zone 7.
If you want to grow crapemyrtle in northern areas, various microclimates and cultural practices can enhance hardiness. Avoid excessive watering, pruning, or fertilizing in the fall which forces new growth that will not have time to harden off and is likely to be killed by winter cold. Avoid planting against south-facing walls which hold and radiate heat and may cause premature breaking of dormancy during brief winter warm spells. Established crapemyrtles fare much better than younger ones when it comes to withstanding the vagaries of winter weather because of their increased trunk size and decreased tendency to grow rampantly late in the season. Keep in mind that crape myrtles are heat loving shrubs/ trees, and may not bloom as well in cooler climates.
Hope this info helped.)