The calendar still says it’s summer, but you may have had to get out your rake to clear dried-up leaves from your lawn. Why does it look like fall already?

“Trees are reacting to this drought by protecting themselves,” said Jason Lubar, associate director for urban forestry at the Morris Arboretum of the University of Pennsylvania. “They do that by letting go of their leaves when it's very hot and very dry.”

With temperatures in the 90s for days at a time, Lubar said leaves start to shut down.

"If they shut down for long periods, the trees realize that they are under stress, so they send out hormones that cause the leaves to drop," Lubar said.

It’s a natural strategy that plants have developed over eons to survive intermittent drought. 

“Leaves are falling as trees try to preserve carbohydrates,” said Vince Marrocco, director of horticulture at the Arboretum. 

“When you get hot, dry periods like we’re having right now, plants are trying to take all of those reserves out of the leaves before the leaves die because there’s not enough water,” he said. “For the month of August, we received less than two inches of rain and had above normal temperatures for 21 of 31 days.  We also had an eight-day and a 12-day heat wave. This combination of prolonged heat and drought are particularly difficult for plants."

The Pennsylvania Department of Environmental Protection has declared a drought watch for 36 counties, including Philadelphia and surrounding counties. Residents are being asked to cut their water use by up to 10 percent, or a reduction of three to six gallons per day.

The DEP recommends watering your lawn only if necessary during this drought watch, but Peter Fixler, chief arborist at Morris Arboretum, said it’s better to water your trees, not your lawn.

"Maybe save watering the lawn to save your trees," he said. "Lawns go into dormancy. Trees will not. They'll just kind of desiccate. They may go on a downward spiral if you don't keep on them enough. That could be a lot more expensive than replacing a lawn."

This dry weather may have another effect on trees – it may shorten the colorful fall foliage season. The National Oceanic and Atmospheric Administration says drought conditions during late summer and early fall can trigger an early shutdown of trees as they prepare for winter. This can cause leaves to fall early without reaching their full color potential.