By Corinne Yoshihara, UC Master Gardener of Napa County
As I traveled through Napa Valley this spring, I made a special effort to note the progression of colors on trees lining my route. Color change (or the appearance of color) occurs every year, yet I never tire of it. It's one of my favorite aspects of spring and it's especially noticeable on trees. I enjoy watching the transformation of stark brown branches to those with pops of whites, pinks and, now, green.
Having had a good winter rain, the flowers were particularly abundant this year (as are the weeds, sigh). I first noticed the white flowers of evergreenpear trees in February. Their white flowers were striking against the grey sky, made even more pronounced by the scarcity of other flowers. Pollen-covered anthers lent a charming speckled appearance to the white petals.
With the landscape being so bare, it was also easy to spot the large white flowers flushed with purple of the saucer magnolia, one of the early blooming hybrids of magnolia. The creamy white flowers of southern magnolia didn't make their appearance until May.
When the February rains took us into March, I noticed more arboreal change. Plum trees were covered with soft pink or white blossoms. Bare maple trees sported flowers resembling little red or yellow-green (red maple and silver maple, respectively) balls of bristles. Also around this time, bright pink pea-like flowers covered the branches and trunk of the redbud tree displaying a trait called cauliflory, or “flowering on mature trunk.”
By April, many more trees were in bloom. I saw striking red clusters of the red horse chestnut flowers, white fringe-tree flowers that resembled dragonfly wings, and crabapple blossoms in colors of white, pink and red.
One tree in particular was especially beautiful in April, the dogwood, one of my favorites. Usually blooming under the dappled light of shade trees just beginning to leaf out, the dogwood's tiered branches are sheathed in white, pink or red depending on the variety. The “flowers” are actually four bracts or modified leaves and the actual flowers are tiny, forming a rounded yellowish cluster in the center of the bracts.
Leaves were out on many trees by May, late spring. It seems like they slowly peep out of their bud covers and then, while you're distracted, cover the entire tree in fresh, unmarred chartreuse. Still, I was able to pick out the flowers of tulip, cherry and hawthorn trees.
The opening of buds raises a lot of questions for the biologist-at-heart. Here is one of mine. Did you notice that many of the earlier flowering trees had no leaves? This behavior of flowering before leafing is called hysteranthous. Although both flowers and leaves begin as buds, flower bud opening is not tied to leaf bud opening.
There are even differences in the times of flowering and leaf unfolding between tree species. Pear, plum, redbud, dogwood, magnolia and cherry trees, for example, all flower prior to leafing. On the other hand, crabapple, horse chestnut and tulip trees have both leaves and flowers at the same time.
Moreover, leaf and flower bud openings can vary within a genus. For example, for the maple genus (Acer), bigleaf maple flower buds open at the same time as its leaf buds; silver maple flower buds open when its leaf buds are still tightly closed; and sugar maple leaf buds open but not its flower buds.
Since flower and leaf bud opening behaviors are not tied, the mechanisms that control them must be distinct. In addition to climatic (chilling, light, heat) requirements, there is a genetic component to flowering. It gets complicated!
Whatever the mechanism may be, it appears that time of flower or leaf opening confers advantages to trees. Trees that produce flowers first, place all of their stored energy into the important process of reproduction before other trees have a chance to leaf out and grow. Also, since flower production, like leaf production, requires sunlight, early flowering ensures that leaves don't block sunlight, or interfere with wind pollination. Masses of flowers may also attract more insect pollinators.
Next time you're outdoors admiring nature's beauty, pay attention to the little things happening around you. You may come up with your own questions. Bring them to the Master Gardeners Tree Walks and share them with us.
The UC Master Gardeners are volunteers who provide UC research-based information on home gardening and answer your questions. To find out more about upcoming programs or to ask a garden question, visit the Master Gardener website (http://napamg.ucanr.edu) or call (707) 253-4221 between 9 a.m. and noon on Mondays, Wednesdays or Fridays.