Up in the Leaves

Believe it or not, November is prime tree-planting season in New York City. For co-op and condo boards with communal rooftops or other outdoor spaces, now is the time to give their green thumbs a workout. 

Shira Boss and her husband, the certified arborist Bob Redman, run Urban Forestry and Tree Care on Manhattan’s Upper West Side. In March, Boss published a children’s book called Up in the Leaves, which tells the story of Redman’s youthful passion for building tree houses in Central Park.

The couple offers a short list of do’s and don’ts for the planting and care of trees on city rooftops and terraces:

Tree Selection

Since trees can get heavy and space is usually tight, don’t choose a tree that can grow 60 feet tall, such as an oak. Dwarf varieties are recommended, as are birch, crape myrtle, and Japanese maple. 

Containers

Square containers are preferable to round ones because they’re less likely to tip over in high winds or if the tree gets tall and top-heavy. Containers should be on wheels so they can be moved for roof maintenance or repairs, and for better drainage. “If they’re elevated, water doesn’t pool,” Boss says. “If a tree’s feet stay wet, it can kill the tree.”

Care

“Trees in containers dry out faster than trees in the ground, so watering can be a big task,” Boss says. It’s advisable to have a reliable water source on the roof because the container needs to be watered thoroughly and regularly, as often as every other day in hot weather. Automated drip irrigation systems are available. In the co-op where Boss and Redman live, there’s a sign-up sheet for shareholders to take turns watering the trees and other plants on the roof. A couple of inches of mulch, such as shredded cedar bark, will retain moisture.

Size Matters

If you’re importing a tree from a nursery, Boss and Redman advise you to think small. Young trees are cheaper, easier to transport, and they’ll be hardier if the roots can expand inside the container. As trees mature, it’s sometimes necessary to trim the tops and the roots.

Prepare for Heartbreak

“Anything you plant on a roof is a risk,” Boss says. “You have to be prepared to lose stuff. It’s always heartbreaking, but if you started small and haven’t spent thousands of dollars, it’s not going to be so bad. And it’s kind of fun to shop for a new tree.”