The second flush of growth

We think of summer as the “growing season” and we are used to our garden plants growing all summer. For most trees, though, growth takes place mostly in the early spring. Most trees in temperate regions produce a single flush of growth – the bud opens, leaves that were formed the previous year expand to their full size, and growth stops. Shoot growth is often finished for the year by June, even earlier in some trees. In a typical year, a new bud will form and rest until the following spring. Some of this is related to rainfall, with spring rains often giving way to a dry summer.

This year, though, we have had episodes of heavy rain in June, and this has stimulated many trees to produce a second flush of growth. We can easily see the second flush in many trees because the second flush is pale, lacking the deep green appearance of the first leaves. This is a result of nitrogen depletion. The first flush of leaves take advantage of available nitrogen that has accumulated in the soil over the winter. By the time of the second flush, nitrogen has been depleted. Over time, the leaves will accumulate more nitrogen and become darker green.

What is especially interesting about this years second flush is that this is the fourth year in a row that we have seen a second flush in trees in central Kentucky. I used to think of this as a rare phenomenon, seen only in unusual years. Our steady increase in rainfall means that second flushes seem to be much more common.

Keep an eye on trees in your neighborhood and see if there are second flushes.

Bur oak growth
Two flushes of growth in bur oak. First flush (white arrows), second flush (yellow arrows)

American elm
Second flush, American elm

Hackberry growth
Second flush in hackberry

American elm
Second flush, American elm