Maximum Yield Australia/New Zealand January/February 2020 | Page 59

6
Weedkiller
Sometimes, we are our plants’ own worst enemies.
A slip of a spray gun, the wind blowing in the wrong
direction, and a splash of herbicide lands in the
wrong place. Result: yellow leaves. Protecting nearby plants
with sheets of cardboard helps, but it’s better for them and the
environment not to spray at all.
Herbicide damage can happen indirectly. Aminopyralid is a
herbicide commonly used on grazing pasture and can end up
contaminating manure, causing serious damage when used,
unwittingly, by gardeners. Source manure carefully; if you
can’t be sure, a test sowing of susceptible plants like fava
beans will tell you if your batch of compost is clean or not.
Other symptoms to look out for:
• Stunted growth: Stalled growth and yellowing leaves are
classic signs of general broadleaf herbicide damage.
Prune off affected leaves as quickly as possible, and water
thoroughly to dilute chemicals.
• Curled leaves: When seedlings emerge with distorted,
fernlike leaves that curl in on themselves, it’s likely you’ve
been using manure contaminated with aminopyralid. Ditch
affected plants, as they won’t survive. The chemical breaks
down in soil, so after a year compost should be safe to use.
• Brown patches: When leaves yellow in patches which die
back leaving the rest of the leaf healthy, your plants may
have been exposed to a contact herbicide such as diquat,
which kills only where it touches. It’s sometimes used by
farmers to clear fields prior to sowing. Rinse thoroughly
with water and if the plant is only lightly damaged it
should recover.
“
WHILE YELLOW LEAVES
are a cause for concern,
you’ll need to find other symptoms
before you can decide on a treatment.”
7
Normal Aging
Yellowing leaves look alarming — and sometimes
they are. But just occasionally, plants need to lose
a leaf or two. We’re familiar with the fall leaf drop,
but what’s less well known is that evergreen trees do this
too, shedding a few older leaves gradually through the year.
Brassicas (like cabbages, calabrese, and sprouts) also do this
naturally as they mature.
As long as you can’t see any other more sinister
developments, such as wilting or the loss of young leaves,
there’s nothing to worry about. This is a perfectly natural
process, known as senescence, in which the leaf stops
growing or photosynthesising as the plant no longer needs
it. It loses its green colouring, dies and then drops away,
leaving the rest of the plant to grow on as normal. It’s worth
picking up the yellowing leaves, so they don’t become
havens for fungal diseases like botrytis — but otherwise,
you don’t need to do a thing.
When you spot that yellow warning light among your plants,
it’s worth sitting up and taking notice. But you’ll need to look
closer to find out what’s really ailing them. Once you’re confi-
dent of your diagnosis, act quickly, apply the right medicine,
and in most cases, the patient will make a full recovery.
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