YELLOW
ALERT
7 REASONS FOR
YELLOWING LEAVES
by Sally Nex
Yellow leaves are part of nature’s process but
yellowing can mean trouble for your plants.
Sally Nex examines seven reasons why leaves go
yellow and ways to get your plants back on track.
ellow is an alarming shade to find in the greenhouse, especially
when you’re not expecting it. From a limey pallor to the startling
Y lemon
of truly sick foliage, yellow leaves glare out like warning
signs against the lush deep greens of healthy foliage.
The trouble is, yellow leaves are the plant health equivalent
of a headache — a general symptom that could mean anything.
Throbbing temples in humans can be caused by a brain tumor or
just a good party the night before. To get to the bottom of exactly
what’s wrong, a doctor must investigate further, and it’s the same
with plants: while yellow leaves are cause for concern, you’ll need
to find other symptoms before you can decide on a treatment.
Essentially, when a leaf turns yellow, it is dying. We’re all familiar
with the process: it happens every fall, as deciduous trees get
ready to slip into dormancy for the winter and shed their foliage.
Previously green leaves turn spectacular shades of red, orange, or
yellow as they die, then detach from the tree and float to the ground.
The fall leaf drop is when trees reveal their ‘true’ colors. The
green pigment in most leaves is a chemical, chlorophyll, which
enables the plant to photosynthesize and turn sunlight into sugar.
As the leaf dies, a layer of cells form along the base of the stalk
attaching it to the plant, effectively sealing off the pathway of
sugar from leaf to plant. The leaf stops photosynthesizing, levels
of chlorophyll drop, and the leaf reveals the underlying pigment —
usually yellow carotenoids (also responsible for orange carrots and
yellow corn). Eventually, this color also fades and becomes the
brown of dead tissue (or the necrotic black of rot).
So, a yellowing leaf is a natural process. But in a healthy
plant in midsummer, when leaves should not be dying, it’s also
a signal something is up. There are lots of possible causes,
so we’ve put together a symptom checker of the seven most
common reasons for yellowing leaves to help you decide what’s
wrong, and what to do about it.
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