|Yellowing leaves on California coffeeberry (Frangula californica)|
It’s May, June or early July. The days are warm and the garden’s transitioning from spring to summer. Suddenly, you notice yellow leaves on your evergreen shrubs. If the shrubs have been thriving all spring, the appearance of yellow leaves can be surprising and alarming.
Take a deep breath. Then carefully examine your shrub. Are the yellow leaves larger and older? Lower on the branches (closer to the trunk)? Are the yellow leaves scattered throughout the foliage (not concentrated on a single branch)? Are healthy new leaves emerging? If so, your shrub is likely exhibiting a normal seasonal process – summer leaf drop.
|Note that the larger, older leaves are yellowing|
on this 'Ray Hartman' ceanothus
Evergreen plants lose their leaves, just like deciduous plants. But they lose them less frequently – and not all-at-once. Shedding old leaves is but one way that woody plants conserve their resources. Old leaves are often less productive. They are also more likely to be unhealthy. In short, senescent leaves become a drain on the plant. They simply require more resources than they make, an unfavorable cost-benefit ratio.
And so, evergreen plants shed their old leaves, but not haphazardly. They usually recycle mineral nutrients and plant chemicals before they jettison an old leaf. The declining leaf then produces less green chlorophyll, becomes yellow (or orange) and ultimately separates from the branch at a special site called the abscission layer. The process is relatively safe and painless for the plant; the abscission layer ‘walls off’ the leaf scar on the branch, preventing disease. And the senescent leaf simply drops off – its work complete.
|Older toyon leaf turning yellow & red. Note disease.|
Evergreen plants drop their old leaves at different times of the year. Some lose them, a little at a time, throughout the year. But many large California native shrubs, particularly those from the chaparral, lose their leaves in late spring/early summer, before the dry season begins in earnest. This allows them to channel their energy into summer growth and drought avoidance. So summer leaf drop is perfectly coordinated with our challenging mediterranean climate.
|Coffeeberry leaves provide summer leaf color.|
So what’s a gardener to do? If you have a big garden event that requires an immaculate garden (a garden wedding? a visit from the queen?) then gently remove the leaves just prior to the event. This will improve the appearance and won’t harm the plant. Otherwise, sit back and let nature take her course.
Enjoy a bit of ‘summer leaf color’. Let the leaves fall naturally, creating a native leaf mulch to support your many soil creatures. Savor the yearly changes associated with our natural heritage. Summer leaf drop is, after all, part of the cycle of seasons in a California native garden.
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