Our street trees needs water. Trees need approximately 10 gallons of water per diameter inch. A new street tree, in most cities, is 2 inches in diameter, thus your newly planted street tree needs 20 gallons of water per week, especially during dry, summer months. Casey Trees in Washington, DC recommends 25 gallons per week for new trees. It is not necessary to water your tree if it rains at least 1 inch during the week, according to Million Trees NYC. Otherwise, water weekly between May and October. Casey Trees' minimum is 1.5 inches of rainfall.
Gator bags slowly release water. Learn about the Treegator's capacity. A DIY alternative to the Treegator is to use a perforated bucket (I have read that you should make the holes in the bottom of the bucket but you could consider making the holes at lowest point on the sides of the bucket). Yet another option is a DIY water bag; instructions at Instructables.
2. Aeration loop
Many street trees are planted with aeration loops installed around their root balls. The loops are perforated and wrapped to prevent clogging of the exchange holes. Pouring water into the loop directs water to the tree's root ball. (The growing root ball should displace the loop.)
|Watering a tree with a hose via Sacramento Tree Foundation (source)|
The hose seems like the classic way to water a tree. If there is a water source near your street tree, why not hook up a hose and set it on a slow drip. For how long? Place your hose in a container whose size you know and see how long it takes for the container to fill. For example, if it takes 5 minutes to fill a 1 gallon container, then it will take 100 minutes to water a 2 inch diameter tree. (Recall that the rule of thumb is 10 gallons of water per diameter inch.) What can you do with that time? How about reading a book? Need a recommendation? Check out the The Tree Care Primer by Christopher Roddick.
A note about where to apply the water: if you have a newly planted tree, water somewhere between the trunk and the edge of the canopy, but not against the trunk. If your tree is established, water at the edge of the canopy or closer to the edge of the tree pit.