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Sunday - May 11, 2008

From: Plano, TX
Region: Southwest
Topic: Trees
Title: Mountain laurel with new leaves or new seed pods
Answered by: Nan Hampton

QUESTION:

Each spring, my Texas Mountain Laurel seems to put new leaves only on selected branches (actually trunks), and put on seed pods on other trunks. It seems to be mutually exclusive: trunks with new seed pods get no new leaves, and visa versa. Is that normal? The tree in question is about 5-6 years old and seems healthy otherwise.

ANSWER:

You are very observant! Producing flowers and fruits (seed pods) and producing new leaves are both very energy-costly activities, so Sophora secundiflora (Texas mountain-laurel) does one or the other on a particular branch, but not both. The tip of each branch, the terminal bud, holds the primary growth areas of the plant, the apical meristems. The growth will be new leaves unless the cells of the apical meristem get a signal to form flowers instead. The signals for flowering include the maturity of the plant, the photoperiod, and temperature. Various plant hormones are involved, also—for instance, auxin for plant growth (as well as other plant activities) and florigen for flowering.

Some trees either add new leaves exclusively on all branches or bloom exclusively on all branches, but yours is not abnormal with some branches having new leaves and others bearing flowers and fruit. After I read your question, I did a survey of the mountain laurels in my neighborhood and elsewhere around Austin and saw many with new leaves on some branches and fruits on different branches of the same tree like your tree, as well as some with only seed pods on all branches or only new growth on all branches.