Betula nigra, the river birch, is a very common shade tree in the Midwest. This is especially so since the introduction in 1979 and gradual recognition of the Heritage River birch cultivar (Betula nigra ‘Cully’), which culminated in it winning the 2002 Urban Tree of the Year award as determined by responses to an annual survey in arborist magazine City Trees. This magazine serves as Journal to The Society of Municipal Arborists (SMA) and it’s readers select a new tree to honor each year.
The three most common birch species grown in urban environments in America are paper birch, European white birch, and the heat-tolerant river birch. River Birch, or Betula nigra, seems to be the only birch truly adapted to the hardships of urban conditions. It is also the only one that is resistant if not practically immune to the bronze birch borer, a scourge of white-barked birches.
Dwarf River birches for urban settings
While river birches are some of the best birches for urban settings, they are also LARGE trees often reaching 40 to 70 feet high. This makes them a bit large for a lot of garden uses. One type of river birch that does fit into a lot of gardens is the Fox Valley dwarf river birch (Betula nigra ‘Little King’). This tree was discovered and introduced in the late 1970’s by Jim King of Oswego, IL and in 1991 promoted through the Chicagoland Grows program. It is a dense compact growing tree with great exfoliating bark that only grows to 10 to 12 feet after 15 years or so.
This dwarf river birch can grow into a dense shrub-like form that so effectively blocks out light that the entire inside “dies out.” That is how this one appeared before I thinned it, as exhibited by this photo.
As you can see in the photo about, in the summer you cannot even see past the outer shell of the plant to its attractive exfoliating bark.
Pruning the dwarf river birch
By selectively removing branches from each of the five main trunks, I was able to quickly thin out the canopy.
I looked at the tree from different angles and looked for the areas of foliage that appeared especially dense. I then went back to these areas and thinned some more branches out. I then looked at the plant again and this time from the outside of the tree, selectively thinned out the clumps of whorls of branches. This was so that instead of 6 small branches occupying an area, only 2 or 3 did. These were much smaller branches I was cutting, perhaps only ¼” thick.
When pruning birches including dwarf river birches, it is best to avoid Spring and Fall pruning when the sap is flowing as this will stain the branches and trunks by “bleeding” sap.
A late August or early September pruning is probably best as the leaves have produced most of the energy for the tree that they will produce and the hottest most stressful part of summer is past.
Pruning load is the percentage of foliage that is removed from a tree during a pruning session. Some trees can handle a larger pruning load than others. Birches are sensitive trees. Therefore, I limited the pruning load to 15-20% for this session.
Conventional standards recommend no more than 25% of the foliage to be removed, but at Anderson Japanese Gardens where I work, we routinely remove 50% to upwards of 70% of a tree’s foliage with no ill effects.
Keep in mind those trees have been trained every year since they have been planted and have adapted to this level of pruning. They are also not 70 foot high shade trees. But instead have been kept for the most part under 25 feet, so they do not have the structural requirements of a large shade tree.
While this dwarf river birch was not heavily trained as would occur in a Japanese garden, by exposing more of the exfoliating bark and also opening up the canopy to allow more light to shine into the tree’s interior, I think this subtle pruning resulted in a nicer looking and healthier tree. What do you think?
For more information on this dwarf river birch see http://www.chicagolandgrows.org/downloads/foxvalleyriverbirch.pdf
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