Chapter 77 Betula nigra
77.1 River birch
Clump of four river birch trunks. River birches usually form multi-trunk clumps of 3-5 stems. Link to original image.
77.2 Soil: Moist Deciduous Mix
Ratio: 4 parts bark, 1 part grit
Add-ins: dolomitic limestone (~5 lbs/yd), Tree-Tone (15 lbs/yd)
pH: ~6.5 (slightly below neutral)
Moisture: Average to moist. River birches grow faster when they have ample moisture. Prefers moist, acidic, fertile soils.
Light: can tolerate some shade, but does best in half-day sun to full sun.
77.3 Cultivation Notes
I have only seen a few river birch bonsai, which is a shame because they offer several advantages. They are North American natives; they are relatively fast-growing; they naturally have shallow roots; they adapt to a range of soils; the flaky bark develops early; and they tolerate top pruning well.
77.3.1 Potting
Repot in spring.
77.3.2 Pruning
River birches do better with less severe pruning done over several sessions or seasons. They do not tolerate aggressive trunk pruning or topping. Never remove more than 20% of the canopy mass.
Do not prune in spring when the sap is running, as they bleed excessively. Prune major branches in late summer or early autumn.
Remove branches less than 2 inches in diameter as close as possible to the branch collar. Do not completely defoliate branches; like conifers, birches will not bud unless there is some foliage to maintain the branch.
Remove side shoots and suckers all season as necessary.
77.3.3 Feeding
Prefers a balanced fertilizer. A good option is Miracle-Gro Soluble Vegetable and Herb Food (18-18-21) diluted 1/4 normal strength. Birches like acidic conditions, so good organic options are Holly-Tone or Azalea-Tone (both 4-3-4).
77.3.4 Pests & Problems
River birches have some susceptibility to aphids, thrips, leaf miners, and iron chlorosis in high pH soils, Very resistant to birch borers, the main cause of death for the other species in this genus..
77.3.5 Winter Storage
Store in mulch. Protect from wind.
77.4 Propagation Notes
River birches are extremely easy to find in nurseries and garden centers.
77.5 Artistic & Styling Notes
Close-up of river birch bark. Even very young trees have flaky bark. Link to original image.
It can be trained as either a single trunk or multi-trunked tree, but looks more attractive as a clump style of 3-5 stems.
77.6 Additional Information
77.6.1 Natural History
River birch is native to the Southeastern US. It also is the only birch species that is adapted to warmer climates. All of the other species in this genus struggle in our summer heat, and do not get enough winter chill to achieve full dormancy.