If trees are dropping their leaves in the heart of summer, it's a sign that something's wrong

For a number of reasons, it’s not good that your street tree is dropping leaves.

First, it makes a mess, but you already know that. Second, the tree is showing you that it is under stress, which is unhealthy for the tree and makes it more susceptible to insects and diseases. And third, that tree is not working at its full potential to clean the air that we breathe.

For the most part, there are two reasons a street tree will drop its leaves midsummer.

A Lack of Water Can Cause Trees to Drop Leaves

The most common reason that a street tree will drop its leaves prematurely is a lack of water. But even if you watered it regularly, this might not be enough.

Watering a street tree is different from irrigating any other plant in your yard. The tree’s roots are possibly growing under hot pavement and cement sidewalks, both of which retain heat and block water from being absorbed into the ground and becoming available to the roots of the tree. Often, you’ll see a little cutout around a street tree, and many people think that this is the place to water. Not so much.

A shade tree doesn’t have a lot of roots right around its trunk, so watering in that cute little cutout is not going to quench its thirst. The roots can span many metres; therefore, a deep watering of any lawn or soil up to roughly 30 ft. (9 m) from the trunk will greatly assist the tree in acquiring moisture. One thorough, deep watering of the area for an hour or so a week should provide it with enough water so that it doesn’t drop leaves in mid summer, assuming that a lack of moisture is the issue.

If your tree has struggled through a drought period, it is important to know that recovery will take some time, even if you start watering immediately. If the tree became excessively dry, leaves will continue to drop, because many of those leaves will have suffered internal damage beyond repair. The extent of cell damage to each leaf will determine its chances of recovery.

So even with regular watering after a drought, your tree still could have a period of a month where affected leaves continue to drop. This should show the importance of regular deep watering throughout its growing period and, more particularly, as the weather through summer starts to warm.

Another gardener’s tip is to jab your garden fork through the surface of the lawn or soil in and around the drip line of the tree once a month or so, from June through August, to allow water to better soak into the soil (as opposed to just running down the street into a culvert).

Insert that garden fork at least 6 in. (15 cm) deep into the soil dozens of times all around the tree’s absorption zone; this will also benefit any surrounding garden and lawn.

Infestation

Insect infestations or fungal or bacterial diseases can also damage the health of the leaves on your street tree. An aphid that sucks moisture and nutrition out of the leaves will slowly deteriorate the leaf until it is compromised to the point of no return – or, better put, to the point of return to the earth.

Just Ask Wim

It’s good to take a regular close look at your shrubs and trees – a plant may appear uninhabited from afar, but a more up-close and personal inspection may reveal it to be infested. If that’s the case, have the foliage diagnosed by your local garden-centre expert so that you are able to control the problem before serious damage is done.

Which fruits and vegetables grow best in patio pots? When is the best time to cut back rhodos? These are just some of the 100+ burning questions that garden expert Wim Vander Zalm answers in his frank, friendly and often funny bestselling new book Just Ask Wim! Down-to-Earth Gardening Answers.