Growing Trees in Boxes

Approximately 25 years ago a city that I am familiar with planted a number of trees in insulated boxes and placed them along the downtown areas.

Evergreens lasted 1 to 3 years and looked like garbage the whole time. Deciduous trees lasted longer but it was obvious that they where struggling.

These trees were moved off the streets for the winter and stored in nearby vacant lots or parks.

The International Society of Arboriculture reports that the average life span of trees planted in sidewalk pots is 7 years; these potted trees cannot be expected to do more.

I would advise you not to use smaller growing trees; these slower growing species will have a more difficult time surviving than more vigorous species.

The problems faced by these trees are similar to trees growing in areas with exposed bedrock and very small pockets of soil. They must also deal with exposure in the winter and summer drought. Simply look around your area and note what trees have survived these conditions. My guess is you will find native species that normally would have grown to 50 feet or more but which have been stuck at 15 to 20 feet for decades.

I recommend using the cheapest and most easily obtainable native species that you can find, in a size that can be handled easily but large enough to make an immediate impression. They should not be larger than 60 mm or 2 inches. Even Acer negundo (Manitoba Maple), usually considered a weed, has a place here. The trees should be tall enough that no trimmming will be required to 8 feet of headroom over the sidewalk or patio when they are in boxes.

If you dig bare rootstock in the area (before leaf out in the spring) ensure that the roots are covered with moist material immediately to prevent the death of feeder roots!

Have drain holes and some gravel in the bottom of the box and then a layer of soil mix sufficient to place your trees on to obtain the desired height in the box. Have a second truck on hand with more soil mix to cover the root system, then water ASAP with a water-soluble starter fertilizer solution, such as 10-52-10 or 15-30-15, at rates recommended on the label. Let the water soak in and then top up the soil and water again. Leave room for a 3 to 4 inch layer of organic mulch, such as wood chips, to help insulate the soil and conserve moisture.

If the trees are not stable in the box, place 2 stakes in opposite corners and use a soft material such as cloth to tie the tree, just until it is stable. A soil mix similar to that used for flowerpots and hanging baskets will help to prevent compaction and provide some nutrients.

Here is one recipe but there may be a more appropriate mix.

• 7 parts garden loam
• 3 parts organic material, such as composted manure, peat moss, etc.
• 2 parts medium to coarse sand 

To each cubic yard of this mix add the following: 

• 2.5 pounds superphosphate
• 1.5 pounds blood or bone meal
• 1.5 pounds sulphate of potash
• 2.5 pounds agricultural lime

As with all potted material regular watering is vital. Use a moisture meter for house plants to check the water requirements daily. Water-soluble 20-20-20 fertilizer may be added to the water at half the recommended strength in alternating weeks.

Jim Althouse

Jim Althouse is a certified arboriculturist with long experience in dealing with urban forest and tree problems and advising municipal authorities on issues related to the maintenance of parks and streetscapes. His e-mail address is jba@althouse.on.ca)