Maximum Yield Australia/New Zealand January/February 2020 | Page 56

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
Y pallor
to the startling 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 tumour 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’ colours.
The green pigment in most leaves is a chemical, chlorophyll,
which enables the plant to photosynthesise 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
photosynthesising, levels of chlorophyll drop, and the leaf
reveals the underlying pigment — usually yellow carotenoids
(also responsible for orange carrots and yellow corn).
Eventually, this colour 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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Maximum Yield
1
Mineral Deficiency
Feeling pale, a little jaded, under the weather?
All are symptoms of anemia, or shortage of iron.
It’s the same for plants: when plants are short
of the minerals they need to thrive, they do the plant
equivalent of going pale — their leaves turn yellow.
Most soils contain a good mix of minerals, but they aren’t
always available to your plants. Acid-loving blueberries
grown in alkaline soil are unable to absorb iron; forget to
feed plants in pots and they quickly use up the nitrogen in
the potting compost and start to starve. Waterlogging and
drought can lock up minerals away from plants, too.
Other symptoms to look out for:
• Green veins on older leaves: Yellowing leaves with
green veins could mean magnesium deficiency,
sometimes caused by over-feeding. Apply Epsom salts
as a foliar spray every two weeks.
• Brown, crispy leaf edges and green veins on young
leaves: This is iron deficiency, common in acid-loving
plants grown in alkaline or neutral soils. Grow in pots of
ericaceous compost instead.
• Spindly growth: Yellowing, weak, slow-growing plants
are often short of nitrogen. Put it right with a good
feed of nitrogen-rich liquid feed, followed up with a
slow release feed and mulch.
• Purple tints: Yellow leaves blotched with purple can
mean potassium deficiency, especially if plants aren’t
fruiting well. Remedy with a dose of potassium-rich
liquid tomato feed.