How to Grow Trees on your Chicago Roof Deck

Living in a condo in the city of Chicago, I do not have much outdoor space to grow trees and other plants. However, I am lucky to have a roof deck on which I have grown a wide variety of plants. With some careful planning, you can grow almost any plant on a roof deck, including trees. If grown successfully, you’ll love the benefits of mid-summer shade and simple addition of nature trees bring to your roof living  space. Here are some considerations for successfully doing so:
 
1.Mature Tree Size
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Be sensible on your species selection, and make sure to choose only small to medium sized trees. You shouldn’t be planting a red oak on your roof deck, for example, as it wants to get over 100 feet tall.  As a tree grows, it not only takes up more space on your deck but also requires more water and nutrients. You will have to consider how wide the branches will get, so that you won’t have to prune them back from walkways and seating areas multiple times per year.
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2.Container Type
 
Heavier containers (wood, clay, etc.) are a better choice for rook decks than plastic as they are more resistant to tipping over in strong winds. Planting multiple trees in one large container is better for your trees that individual small containers. In a large container, trees can share rooting space and soil.
 

Several trees growing in a large container share soil. Source: https://www.houzz.com/photo/9439809-nyc-townhouse-garden-roof-terrace-stone-patio-bench-container-plants-birch-contemporary-deck-new-york

3.Sunlight
 
Your roof deck likely gets a LOT of sunlight. My deck gets basically 100% sun exposure. Therefore, you’ll need to select plants that are adapted to such sunny conditions. Choosing shade-loving plants can lead to severe water shortages and foliage scorching, which can lead to other tree health problems in the future. Another consideration is that, so long as you provide your trees with their needed water and nutrients, the excessive sunlight on your roof deck will cause your tree to have more substantial growth year over year than trees of the same species grown on the ground,
 
4.Water Availability
 
Your roof deck trees are going to need a lot of water. If your deck is like mine, it has 100% sun exposure. That means trees will need use up a lot of moisture every day they are leafed-out. You’ll need to provide water to your trees even when they don’t have leaves. You will need to determine where you’ll get water from for your plants. The source will likely be a hose-accessible spigot or water from a faucet inside your home.
 
I do not have water access on my roof, so I am forced to carry multiple gallons up from my condo. I sometimes have to do this 3 times per day on a hot mid-summer day. You can also purchase or engineer self-watering containers that will at least store a moderate amount of water.
 

A lack of water can cause leaf scorch, as shown here. Source: https://www.ksuhortnewsletter.org/newsletters/category/leaf-scorch

​5.Nutrients
 
You’ll need several inches of soil to support fulling grown trees in containers. Small trees need a depth of at least 1 foot, and medium trees will need about 3 feet. The available soil should be at least 4 feet in diameter for small trees and 8 feet in diameter for large trees.
 
Place a few inches of natural mulch on the tops of your containers to help with nutrient cycling. Mulch will also help to keep the containers from drying out as quickly.

Soil is the heaviest component of a roof deck planting system. No matter the size of containers or the amount of soil you use, make sure your roof can handle the weight. For containers supporting large trees or those covering  more than a few square feet of your roof, you should consult with a structural engineer to make sure your roof can handle the additional weight. 
 
6.Cold Hardiness
 
You will need to consider the temperature of your specific roof deck, not just the surrounding area. Your roof deck is likely quite exposed to the wind. This can be especially concerning in winter.
 
As you can see, there is a lot to consider when trying to grow trees on  your roof deck. However, with some thorough planning (and a LOT of watering) you can grow a wide variety of trees and other plants that will help make your outdoor space more enjoyable.