Oak Flowers

Oak Woodlands
Home

Seedlings
    Gophers
    Annual Weeds
    Cattle
    Deer

Life in Mature Trees
    "Spanish Moss"
     Mistletoe
     Leaping Lizards
     Diseases, Decline
     Sudden Oak Death
     Insects
     Fire

Key to Oak Species

Restoration
Planting Trees
Climate Change

Oak Flower

Acorns start out as a few cells at the tip of the branch. The end of each twig can have flowers of both sexes and of course, new leaves. With warm, sunny weather in March or April, some cells swell into a vase-shaped ovary topped with tiny, sticky, fuzzy knob, the pistil, ready to capture any wind-blown pollen. The ovary walls become the acorn, and the petals and sepals fuse into what becomes the "cap" on the acorn. Other cells at the tip of the branch elongate and swell into a tiny waterfall of yellow balls, each able to explode to release a cloud of pollen grains. At the same time the ovaries and pollen form, other bud cells divide and grow into crenulated sheaths that swell and stretch into the familiar leaves we recognize. There are ways that oaks avoid self-breeding. Pollen and female flowers appear on one tree at slightly different times, and apparently, pollen from one oak does not pollinate the flowers on the same oak.

On the same tree, other buds grow to produce male flowers. These "catkins" (see below) have many tiny spheres, each opens and relseased thousands of pollen grains that are blown in the air. Some reach the pistils of developing acorns and polinate them. Is it not known how far pollen from one tree can blow and still effectively pollinate female flowers on nearby oaks.

On warm, spring days, sometimes you can write your name on the windows of your car in all the pollen. A sudden gust of wind disgorges yellow clouds from the oak woodland. During other springs, the days can go on andon with constant cold drizzle, washing the pollen into the ground. For wind-pollinated oaks, a cloudy, wet spring can mean most flowers never catch the pollen they need to swell into acorns. A sunny, hot spring often means many acorns that fall. Walt Koenig has studied the relationship between warm spring weather and acorn abundance in the following fall. Studies underway at Hastings show that most oaks have a great abundance of flowers and they seem to develop normally. Generally, there are more than enough flowers to grow into acorns and provide adequate seeds for saplings to sustain a stable population of oaks.

In most years, many trees make acorns. Here is a picture of mature acorn. However, there are some years when acorns are scarce everywhere, and some years when they are abundant everywhere. These cycles of abundance are fascinating and important to many kind of wildlife. For more on this, see the next section on acorns.