Plant of the Week: Planting Trees

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I love all plants but if I had to choose a favorite group it would probably be trees. My fascination with trees is probably some deep-seated primal thing casting a shadow from eons ago when trees were important in our daily survival as a species. Maybe, or maybe I just find their enormity and permanence fascinating. It doesn’t matter much but over a lifetime I’ve planted quite a few and can make some general observations.

The most important observation is that trees take time to develop, so the best time to plant them is now. Fall is the ideal season for tree planting but with container-grown plants we are no longer bound to the seasonality rules of past generations of tree planters. Fall planting allows trees to better establish roots before the stresses of summer, but spring and summer planting work too, but require closer attention to watering. 

Choosing the right location is important but getting it in the ground sooner rather than later so that it will have time to grow is critical. Tree planters must visualize what size and shape trees will assume as they mature and plant accordingly. Height and spread are critical information to consider but few of us have the luxury of spacing trees for the size they will attain at maturity.  Instead it is best to consider some intermediate timeframe, say 20 or so years in the future. 

I’ve been planting the arboretum at the Botanical Garden of the Ozarks and I spaced most of the large shade trees 30 to 35 feet apart. At this distance the tree canopies will close in about 20 to 25 years, creating a boulevard of shade for future visitors. If my goal were to plant free-standing specimens that would never touch their neighbors, the spacing would have to be much wider – say on the order of 80 feet apart. When planting around a home I like to keep trees at least 15 feet from structures but sometimes the mature form of the tree may require a different spacing.

What is the ideal size tree to plant? I’ve planted trees in all sizes – from bare root 1-year-old seedlings to 10-year-old behemoths with 800-pound root balls – and have found that mid-sized trees generally work best for me. Any size will work but the amount of attention to detail varies significantly between the sizes. Tiny seedlings take close attention lest they get lost in the shuffle. Really big trees present an immediate presence in the landscape but avoiding transplant shock and careful attention to satisfy watering needs is critical for establishment.  Medium sized trees establish nicely and grow quickly but in areas with roving deer herds, trunk protection must be provided to keep bucks from rubbing off the bark. 

Trees are no different than other plants in that they grow best in good soil. Well-drained soil that hasn’t been compacted by construction equipment works for most species. Too often, especially around construction sites, the soil structure has been completely destroyed and trees are expected to grow in compacted subsoil with a 4-inch frosting of topsoil that is barely able to support grass and weeds. In such cases breaking up compacted subsoil and adding a thicker layer of topsoil improves the odds of long-term tree establishment.

Colleagues of mine when I worked at the UofA built their research careers around studying soil amendments as they relate to tree establishment. Basically they concluded that while soil amendments may help during the first year or two after a tree is planted such amendments offer no benefit for the long-term survival of the tree. 

If a tree, with its wide-ranging root system, is going to flourish it must do so in the natural soil of the site, not in some small pocket of soil around the root ball. Hence, maintaining the integrity of the natural soil without compacting or otherwise defiling it is critical if the tree will show its true potential.