Pests And Diseases Of Hibiscus
The organic philosophy
The aim of all organic practices - permaculture,
biodynamics, natural farming - is not just to reduce
and ideally to eliminate the use of toxic chemicals
that are harmful to humans, animals, plants and the
planet, but to understand the processes of nature in
order to participate, rather than to interfere.
This means a change in thinking. Instead of
thinking monoculture on a massive scale, think
biodiversity over a smaller area. Instead of thinking
of insects as pests, think
of them as part of the natural order, with the plant-damaging insects kept in check by predators. Instead
of thinking of perfection in ornamental and edible
plants as blemish-free, know
that nature is not driven by glossy advertising.
Think always of the way plants live without interference from humans.
- Food for resistance
- If you feed your hibiscus with compost-good-quality, balanced compost -you can never give
them too much. Conversely, if you feed any plant
with chemical fertilizers, you can easily give them
too much. Too much can kill off all micro life. With
edible plants, too much can contribute to
cancer promoting nitrites, and the imbalance that can
result from overfeeding with artificial fertilizers
can create pest and disease problems. Nitrogenous
fertilizers in particular can be harmful because
they promote rapid soft growth that attracts sucking insects.
Just as mammalian predators pick off the runt of
the herd, so insects will overcome the weakest of the
crop. If you doubt that compost is resistance food,
try an experiment using both organic and inorganic
fertilizers by growing carrots or
cabbage in two lots.
Give one lot the best compost you can and the other
lot a nitrogenous fertilizer, such as sulfate of
ammonia. The latter will quickly be eaten by pests.
Another analogy is stress: stress in humans results
in vulnerability to colds, flu and infectious diseases.
Stress in plants makes them similarly susceptible.
A soil rich in organic matter not only provides
nutrients but inhibits the growth of fungal and
bacterial diseases, such as phytophthora, by
encouraging the natural antibiotic activities of mycorrhizal fungi. And of course, the texture of such
a medium provides hibiscus roots with that high air filled porosity they need. When they are healthy,
well-fed hibiscus shrug off or outgrow pest damage.
- Stability in the garden
- Aim to provide all plants with the conditions they need. Aim for both
lateral and vertical diversity, by matching plants with the conditions that suit
them at every level, from
ground-covering creepers through mid-high shrubs to the top of the canopy.
A stable garden does not contain impossibly
difficult plants from quite different climates and
conditions. Nor does it grow a huge number of similar
plants in isolation from all others. And apart from the
procedure of planting and transplanting, a stable
garden is not routinely dug or hoed. Rather, layers are
allowed to build up in the way a rainforest floor
accumulates. Mulching and layering duplicate
the processes of nature.
A stable garden in which natural pest control is
practiced will always have a constant, but manageable, supply of pests providing food for a constant
number of predators. Both are accepted as part of the
ecosystem. Not all insects are bad -some are
predators, some never cause plant damage, and others never build up their numbers.
- Companion planting for insect control
- Companion planting is a component of the stable garden, but it is not a
cure-all. On its own it is ineffectual: it doesn't rid the garden of pests, but it
does aid diversity and stability. Through releasing
different scents, through utilizing different levels of
soil, and by occupying different garden spaces in
certain combinations, companion planting can
attract, or repel, specific insects. For example, plants
which attract aphids and are good companions for
hibiscus include the annuals feverfew,
nasturtium, and Hyssopus officinalis.
Some hibiscus pests are worse than others. The larval
grubs of garden butterflies can chomp their way
through many a hibiscus leaf, but their harm is mostly visual. There are grubs which live off the foliage of
shrubs and soft-wooded trees and other grubs which
usually hatch out simultaneously. This means that if
you find one, you'll find more. Watch closely for them,
especially in the early morning, and if you discover
them in the early stage, you may be able to pick off and destroy the whole batch.
- Chewing insects
- Beetles too can be picked off. Apart from the flea-like
bronze beetle (Eucolaspis brunnea), which leaps too
quickly to be caught, most beetles can be hand
removed if the bush is not first disturbed.
Snails and slugs, though voracious chewers, are a
threat only to fresh new leaves, mainly on young
plants. The best natural control is plentiful bird life.
Other chewing pests include grasshoppers and
katydids, those bright-green, well-camouflaged, flying
bugs. Stick insects are also chewers, but fortunately
don't often bother hibiscus. Excelling at camouflage
on the bush, the females lay eggs which drop to the
ground and emerge as nymphs in spring, climbing up
the stem to feed on new leaves. A simple way to
prevent them getting there is to band the base of the
trunk or stem with a ring of grease - try Vaseline - that
stops them in their tracks.
- Sucking insects
- Sucking insects may be less conspicuous. Although
the green vegetable bug, Nezara viridula, and its
immature form with the colorful mosaic back are
often easy to spot, most sucking insects are a lot
smaller. Scales are only 1/16 th in (1 mm) long; spider
mites, pinprick-sized pests, can cover the undersides
of leaves; powdery or fluffy mealy bugs are a menace,
and there's always the common aphid. Be alert for
these. Evidence of their presence is white spot or chlorosis of the leaves (spider mites), misshapen buds
(aphids), or sooty mould (by-product of mealy bugs and aphids).
To avoid spraying other than a last resort, try first
squashing these pests by hand or removing them with a strong-pressured hose. Squashing or hose-blasting
mealy bugs will probably not work though, because
they coat themselves and the surrounding surface with
a waxy substance that resists dislodgment. For mealy
bugs, try a paintbrush application of water mixed half-and-half with methylated spirits. Spot applications of
this sort (i.e. using a paintbrush) are always
environmentally safer than wide-coverage spraying.
Thrips can be difficult to deter as they are fast
movers. They are worst in dry conditions, and are not
usually a problem in moist areas. Frequent hosing and
misting can eliminate them. Whitefly infestation most
often occurs in heated glasshouses. It shouldn't be a
problem for the home garden where hibiscus have good air circulation.
- Other pests
- A big problem is gall midge. It lays its eggs in the flower buds, which the
larvae then damage. The buds eventually drop, and this bud-drop is the most
obvious sign of the pest. To rid plants of this pest, remove all dropped buds
or buds that begin to mold. Regularly apply orthene
to the foliage and buds, and diazinon to the soil.
Another problem is Japanese beetle, Popillia
japonica, fond of buds and flowers. Adult beetles can
be controlled by spraying malathion, rotenone, sevin or methoxychlor.
There are other pests of course, most notably,
perhaps, the dreaded hibiscus beetle (Macroura
concolor) -scourge of hibiscus gardens in Queensland, Australia, to which it is fortunately confined.
- Natural controls and non-toxic pesticides
- For most of the above-mentioned insects, natural
controls should be tried. Among natural predators
are hoverflies, which can consume huge numbers of
aphids and also eat mites and scale insects; praying
mantis, which are fully carnivorous and eat aphids,
leaf hoppers, caterpillars, beetles and sometimes
moths; spiders, which besides consuming flies feed on
the larvae of moths and whiteflies; and, best of all,
the ever-friendly ladybug. Ladybugs should be encouraged everywhere. Upon hatching, their
larvae start immediately to eat aphids, mealy bugs,
woolly aphids, thrips and other sap suckers, and they
continue into adulthood to clean up these menaces.
Non-toxic sprays should be used with caution,
not because of harm to humans but because
indiscriminate use can kill both the targeted insect
and its predators. The non-toxic preparations
described here are short-lived and break down
quickly, so they should be used up quickly.
Pyrethrum (make your own from Tanacetum
cinerariifolium, or buy the ready-made produce from
garden centers) is a broad-spectrum insecticide.
Garlic spray (soak 3 oz (85 g) of whole unpeeled
garlic in mineral oil for 24 hours and add 1 pint
(600 ml) of water in which a bar of soap has been
dissolved; strain, dilute by five times, and spray) will
work if it is eaten -it is not a contact spray -and
can kill stink, bugs and the mosaic-backed vegetable
bug. Chile spray (1 cup of fresh chilli peppers pulped
with 1/2 cup of dried chilli and 1 cup of water)
controls caterpillars and loopers, which are also
deterred by sprinkled cayenne pepper. Buttermilk
spray (1/4 cup of buttermilk and 2 cups of flour mixed
with 1 gallon (5 liters) of water) will control mites
when applied to the undersides of leaves every two
days. Onion spray can be good for scale, thrips,
aphids and mites (pour 3/4 pint (500 ml) boiling
water over 2 1/4 lbs (1 kg) of chopped unpeeled
onions and strain; dilute 20 times with water for
spraying). Applications of ash from the fireplace,
provided it is wood ash and dry, can control leaf-eating earwigs, grasshoppers and katydids. Some
gardeners recommend diluted antiseptic solution as a good general insecticide.
You can also try traps. Cut a piece of white or
yellow cardboard the size of your hand, and coat it
with Vaseline. Nail it to a stake or tie to a hibiscus
branch. Insect pests, especially night-flying ones,
will be lured to it, as they are to all light, and will
stick fast. Such a simple device is good for assessing
what is in your local insect population. A "trap"
with a different purpose, that of catching aphids,
can be made from a liquid containing sugar and yeast.
Another non-toxic suggestion is a diluted yeast-
based spread laid around the garden in small
quantities to attract predators such as hoverflies and
lacewings, both of which feed on aphids.
- The soil-borne wilt disease associated
with soils that dry out in summer and are wet in
winter, can kill many ornamental shrubs, including
hibiscus. It pays to avoid the kind of conditions
favorable to phytophthora because affected plants usually do not recover. The
problem rarely occurs where hibiscus are given good drainage and ample compost.
- "Collar rot"
- Visible as a browning discoloration,
often slimy, at the root join, is prevalent where
winters are cool and damp, and drainage poor. You
can try a treatment of removing all affected stems
and bark and painting the discolored area with
Bordeaux paste. Clear the surrounding ground
completely and spray with a strong garlic solution.
Sclerotina rot affects stems at ground level.
Repeated applications of strong garlic or chamomile tea can help.
If your hibiscus has succumbed to tot and
you want to replant in the same spot -inadvisable
but sometimes unavoidable -you must first improve
the site. Start by removing the affected plant and
burning it, then dig up the whole area beyond root
depth and spread, distributing the infected soil in an
unimportant area, such as plugging a hole in the
driveway. Some gardeners advocate lighting a fire
and spreading it over the entire site before refilling,
as fire will kill off any remaining pathogens. The
next step is to infill for drainage by lining the cavity with a layer of pumice mixed with fine gravel or
pumice over coarser stones, adding sand, then filling
the cavity with new topsoil. To be extra-cautious,
use sterilized potting mix.
Besides diseases of the soil, there are fungal
diseases that can affect foliage. Leaf spot is the main
one to watch for. Containered plants may be slightly
more susceptible to it than in-ground plants,
especially when positioned in places with less air
circulation. Dark brown or black spots on leaves
indicate the presence of pathogens which are most
active in wet weather. Heavily infected leaves,
where the individual spots have merged to discolor
the entire leaf, will result in shedding, and shrubs so
depleted can probably only be salvaged by the
application of a fungicidal spray, such as Bordeaux.
Liquid Bordeaux can be bought from garden retailers
or made at home to an exacting recipe of copper
sulfate and calcium hydroxide. Bordeaux paste uses
the same two ingredients less diluted, with skim-milk powder for viscosity.
In general, compost and mulch are the best root-rot preventatives.
Dolomite sprinkled on the area
can also help. An all-purpose spray for fungal
prevention can be made from a "weak tea" of nettles,
horseradish. Even better, a weak
liquid poured off from seaweed soaked in fresh water
has a four-fold benefit: it is a foliar fertilizer, it
promotes frost-resistance, it is an insect repellent
and a fungicide, and the residual seaweed can be
composted or used as a mulch.
Solutions to other problems
- Sparse foliage
- Apart from insect infestation or the general poor
conditions that result from undernourishment, the
most frequent cause of sparse foliage is insufficient
sun. Site hibiscus in maximum sun.
- Crinkled leaves
- Check for aphids or leaf hoppers on the undersides
of leaves. Large colonies of aphids can suck the life
out of leaves before they are noticed. Remove the
affected leaves along with the offending insects. If
the infestation is severe, try spraying.
- Deformed and sickly foliage
- If it is possible or likely that your hibiscus has been
exposed to systemic insecticides, it will quickly
sicken. Hibiscus do not tolerate systemic chemicals,
such as maldison, which are absorbed by the plant.
Good feeding and watering may help it recover.
- Yellow leaves
- Yellowing of leaves at the base of branches and stems
occurs naturally as the hibiscus sheds old foliage. If
yellowing occurs higher up the stem, especially at
the growing tips, it could indicate magnesium or
iron deficiency - trace elements easily added to the
soil. Alternatively, the discoloration could be a sign
of over-watering or over-feeding with chemical
fertilizers, in which case suspend both. Yellowing is
another sign of exposure to systemic insecticides.
- Yellow leaves with green veins
- Yellow leaves with green veins are a clear indication
of iron deficiency. Apply a good feeding of compost
and manure, and scratch in kelp meal or seaweed.
Iron chelate is often recommended specifically to
target the deficiency. Dig this in lightly -never pack
down tightly around the stem. Mulch, kept clear
from the base of the stem, can also help.
- Other leaf discoloration
- Brown margins might be windburn, in which case
watering and light feeding can restore health.
Persistent and prolonged windburn, indicative of a
windy climate to which hybrids may not be suited, is
a problem that might be best solved by replacing
hybrids with wind-tolerant coastal species.
Salt toxicity also shows up as brown leaf margins.
Treat with generous watering. Leaves that turn
brown at the tips, purple at the edges, and become
brittle could indicate a lack of potassium.
- Frost damage
- Once frost has damaged your hibiscus, there's
nothing you can do. If severe, the plant may not
recover at all; if light, it will struggle back of its own
accord to produce new growth in spring. Do not
prune affected parts until the weather has warmed and
new growth is established. If you live where frosts can
be expected every winter, you may consider pot
culture to enable you to move the plant to shelter each year.
- Any weeds growing around a hibiscus are competing
with it for nutrients. Remove them. Remember that
the feeding roots of hibiscus are very close to the
surface, depending on the same top few inches of soil
occupied by weeds. The easiest way to keep the area
weed-free is to thoroughly clear it before covering
with a layer of mulch; the mulch will keep the weeds at bay.
- Lawn encroachment
- When hibiscus are sited in a lawn, an area of about
3 square feet (1 square meter) should be kept clear of
competing growth. Mulching will deter the grass from encroaching again.
- Gray, green or silver lichens occasionally establish
themselves on old mature trees. They won't bother
the hibiscus but may bother the gardener. It would
have to cover and smother the whole shrub before
harming it. However, if you want to remove the
growth, scrub it off with a scrubbing brush or apply Bordeaux paste in winter.
- Many ant species are valuable predators. They only
invade hibiscus if there's something there for them
to feed on, and although the odd ant may be
attracted to nectar, ants in large numbers are usually
after the honeydew excreted by aphids or mealy
bugs. Deal with the aphids and the ants will go too.
Moisture is an ant deterrent -they increase in dry
places -as is pyrethrum spray. Banding the base of
the hibiscus stem with Vaseline will prevent ants from traveling further up.
- Poor flowering
- Few flowers in an otherwise healthy plant may result
from too much shade, or, if sun is adequate, from
excessive nitrogenous feeding which promotes leaf
growth at the expense of flower production. Bud damage may also be a cause.
- Buds but no flowers
- If your hibiscus are under attack from borers or
beetles penetrating the buds before they open, the
ground will show the evidence. The treatment of
removing all buds, both from the bush (including
the tips of branches) and from the ground below,
and burning them, followed by insecticidal
spraying, should reduce the problem, but may not eliminate it.
Bud drop can also result from lack of water during
warm sunny weather; it is important not to let hibiscus dry out.
- Flowers appear late in the season
- The most likely cause of late blooming is sun and
temperature -not enough sun and not warm- enough temperatures.
- Blooms change color
- The hybrids sold today are the result of years of intensive hybridizing, of
crossing one with another, and often backcrossing again to achieve desirable
attributes. As with many such "manmade" garden plants, dominant features of
parent plants or earlier ancestry occasionally appear after they have been bred
out. Sometimes hibiscus revert to colors of an earlier form; sometimes they'll
revert from double to single, from ruffled to unruffled, or from overlapped or
windmill single to plain single. This happens most often out of season, while
true-to-type blooms can be expected in summer and fall.