QThe white birch tree in our yard has four trunks. All the other birch trees in our neighborhood have three trunks, so I'm wondering why ours has four.
AIn nature, paper birches (Betula papyrifera) most often grow as single-trunked trees. Nurseries sell some single-trunked birches, but they frequently sell paper birch as clump forms, which usually have three main trunks. This multitrunked effect is created by the nurseries, not by nature. To achieve a clump effect, birch seedlings (usually three) are planted together in a pot or in a nursery field. As the young trees grow, their root systems intertwine and the bases of their trunks often look like they are growing together.
Birches are grown in clump forms to provide a naturalized appearance, somewhat similar to how they might appear in a natural setting. In such a setting, most birches actually would be single-trunked, but they often grow close together, which might make them look multitrunked. By grouping three birch trees together, the nursery is trying to create the feeling of the North Woods, even though you might be planting on a city or suburban lot.
In your case, I suspect that an extra birch seedling got into the original grouping when it was potted at the nursery. There's certainly nothing wrong with having four trunks, and there's no need for you to do anything to change your tree.
Three trunks is the standard number for clump birches because grouping in odd numbers tends to make the plants look more natural and informal. Remember, birches like a cool, moist root zone, so apply an organic mulch such as wood chips in a wide circle around the base of the tree and provide supplemental water during dry periods in the growing season.
-- Nancy Rose is a research horticulturist at the Minnesota Landscape Arboretum. To ask her a gardening question, call 612-673-9073 and leave a message. She will answer questions in this column only.