If you’ve planted trees or shrubs this year, be extra careful to keep them well watered through the heat of summer. 

A new tree, or any new plant, needs extra help to get enough water because it has not yet grown sufficient roots to collect adequate water from the soil. Even if there has been ample rainfall and the soil is moist, the new tree’s small set of roots is too limited to absorb that moisture. To support its survival and growth, it needs to be watered regularly.

Some 1,100 new trees are growing around the Chicago region this summer thanks to The Morton Arboretum’s Centennial Tree Planting Initiative, which marks the 100th anniversary of its founding. About 1,900 more trees are slated to be planted by the end of 2023. The Chicago Region Trees Initiative has managed the plantings in communities that have the most need for more trees, and local organizations and partners are handling the watering. 

Professionals and municipalities often water new trees with tree bags–tough plastic bags with tiny pinholes in the bottom. When a bag is filled, 15 to 20 gallons of water will very slowly ooze out of the pinholes onto the root zone. 

Tree bags work well because they deliver water very slowly, in exactly the right place. All the roots of a new tree are in a small mass called the root ball, right around and below the base of the trunk, and that’s where the water needs to go. 

You can water a tree slowly and deeply without a tree bag. Turn the hose nozzle down to a flow about the size of your little finger and leave it on the root ball, next to the tree’s trunk, for half an hour or so. 

If you’re watering a new tree with a bucket, it will take 10 to 15 gallons, or two to three buckets full. Pour the water slowly and gently onto the mulch right around the tree trunk.

After a slow, deep watering, don’t water again right away. Give the soil a chance to drain so air can reach the tree roots. If the soil is constantly waterlogged, the roots may rot.

The best way to know when you need to water again is to feel the soil near the trunk. The root ball can dry out in a day or two, even if there is moisture in the surrounding soil. You will need to water more often when it’s hot and less often when there has been a lot of rain. Keep watering a newly planted tree for two to three years. 

To ensure your watering effort goes as far as possible, spread an even layer of mulch 3 to 4 inches deep in a wide circle around the base of the tree. One of the many benefits of mulch is that it prevents moisture from evaporating from the soil, keeping it available to the tree.