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By Sandy Lloyd, Research Officer, Department of Agriculture, and members of the Pest and Disease Information Service
A large number of plants are classed as herbs. Some that were not widely known a decade or so ago are now household names, due to their inclusion in commercially available products that are claimed to have health benefits.
In the backlash of public opinion against conventional medicines, artificial flavourings, artificial perfumes, chemical insecticides and so on, some people may be tempted to assume that anything comprising herbal extracts provides a safe, natural alternative - and that the more they use, the better the outcome will be.
A worrying extension of this idea is the increasing popularity of growing medicinal herbs at home or harvesting them from the wild, in the belief that the fresh form is bound to be an even healthier alternative.
Unfortunately, these herbs could have the opposite effect unless you are certain about which part of the plant to use, how much to use and how to prepare it. There is also the possibility of misidentification which may lead to the wrong plant, possibly a toxic one, being used.
Another problem to be aware of is that the cultivation of certain herbs or their collection from the wild can have a damaging environmental impact.
This Gardennote does not endorse any particular herb for any particular use; neither is it intended to cast doubt on commercially available herbal products. The information presented here simply aims to make home gardeners aware of potential dangers.
Beneficial properties are popularly associated with various herbs but many of the claims are based more on anecdotal evidence than scientific proof, and some books and websites featuring herbal remedies do not mention toxic properties or possible side effects.
A mixed herb bed is a useful garden asset. Few health risks are associated with culinary herbs when used the normal way - that is, in small amounts as flavouring during cooking or as a garnish. However, as is the case with any edible plant, eating excessive quantities may cause harm. This fact should be kept in mind especially if a culinary herb is claimed to have health benefits that may make it even more tempting to use too freely.
For example, parsley (Petroselinum crispum) is claimed to be a digestive aid, but eating excessive amounts may damage the liver and kidneys. It is also important to realise that some products made from culinary herbs are concentrated, so the power of the active ingredients may be unpredictable or extreme.
Common herbs that become weeds
In terms of invasiveness, the main culinary herbs that have earned notoriety are common mint (Mentha spicata) and fennel (Foeniculum vulgare).
Given favourable conditions, mint will take over a huge area of your garden and, once established, can be almost impossible to eradicate. The only sure way to prevent its spread is to plant it in a large pot and stand it on a sealed surface. Brick paving is no good, because roots escaping from the bottom of the pot will infiltrate the cracks.
Because fennel self-seeds profusely, it has escaped from gardens and sprung up on roadsides in Western Australia, from where it is a short step for it to invade natural habitats. If you live near bushland and you grow fennel, cut off the flowers before they produce seed, put them in plastic bags and place in the bin.
Lavender (Lavandula spp) is unusual in that although it is not used in cooking, nurseries often display it beside culinary herbs and it has long been a component of the traditional herb garden. Although occasionally used in some desserts today, lavender is still regarded as primarily a fragrant herb used in home-made sachets, manufactured perfumes and essential oils. It is also a popular ornamental.
Unfortunately, one kind of lavender, topped or Spanish lavender (L. stoechas) is very invasive, so if you have it in the garden and you live near bushland, dead-head it constantly to prevent seeds escaping, or grow a less invasive species.
Because of their claimed effects, some culinary herbs are used as companion plants - mainly in vegetable gardens but also sometimes in flower borders, and on a larger scale in permaculture.
Those that are claimed to attract beneficial insects include hyssop (Hyssopus officinalis) and chervil (Anthriscus cerefolium), while borage (Borago officinalis), dill (Anethum graveolens) and sage (Salvia officinalis) are among those that are claimed to deter insect pests.
Whether or not these claims are true, growing culinary herbs as companion plants cannot do any harm. However, the next step some people take is to make their own pesticide sprays from certain herbs - culinary and non-culinary - in the belief that anything derived from herbal sources must be safe.
This is incorrect. Apart from garlic spray, which appears to be well regarded, all other herb-based home-made sprays may be unreliable and therefore are not recommended.
Potentially dangerous garden remedies
A spray made from horsetail (Equisetum arvense and E. hyemale) is claimed to be a preventative fungicide, but the plant is toxic and invasive. When it grows in pastures and horses eat it, they develop a vitamin deficiency that leads to a condition known as equine staggers which can cause death through heart failure. Horsetail is a Declared Plant that landholders are obliged to eradicate so do not grow or experiment with it.
Artemisia or wormwood (Artemisia spp) is claimed to kill fleas. It has been grown in hen coops, the theory being that the chickens rid themselves of fleas by pecking the leaves and bathing in the dust where the spent leaves have fallen.
Whether or not this claim is true, it is risky to make your own insecticidal spray from artemisia or other herbs listed by herbal or permaculture books or websites.
Many such herbal sprays are so toxic that, just like chemical pesticides, they require a withholding time before you can harvest fruit or vegetables. Even if you follow a recipe you cannot be sure of the strength of the spray.
For example, some recipes recommend using a certain number of leaves, without specifying the size of a typical leaf or indicating whether the active ingredients are more potent at certain times of year.
The making and use of home-made herbal medicines - for either people or pets - is not recommended, because claims of health benefits are so often countered by claims of harm.
For example, comfrey (Symphytum officinale) and Russian comfrey (S. x uplandicum) are recommended by herbal books and websites, yet there is also research to suggest these herbs have high levels of alkaloids that may damage the liver.
White horehound (Marrubium vulgare) is also claimed to be beneficial, yet it is known to be toxic to people and livestock. It is a Declared Plant in Western Australia. Do not grow it.
Even herbs that in recent years have become well-known through commercially available products - like St Johnâ€™s wort (Hypericum perforatum), which is also a Declared Plant, and echinacea (Echinacea spp) - should not be used from the garden or the wild, for several reasons.
Since growing conditions can affect the quality of herbs, the active ingredient could be either too low - and therefore ineffective - or too high and therefore toxic. Commercial crops of medicinal herbs are grown in optimum conditions to encourage consistency of product. This is not possible in the home garden, due to variable factors.
Remember, too, that active ingredients are found only in certain plant parts, while other parts of the same herb may be toxic, so using the wrong part may be either ineffective or downright dangerous. Also, there may be big differences in potency between fresh herbs and those that have been dried, processed or stored.
Another reason to avoid making your own medicines from fresh herbs is that some have side effects that you may not know about. For example, eating fresh St Johnâ€™s wort may cause some people to become photo-sensitive and much more vulnerable to sunburn.
Although it is in the same genus as edible mints (Mentha), pennyroyal (M. pulegium) must not be confused with them. Historically it was used to induce abortion, yet amazingly it still features in some supposedly beneficial herbal remedies.
The essential oil of pennyroyal is claimed to combat fleas. However, keeping it in the house could be risky if you have children who might be tempted to taste it because of its minty smell. Pennyroyal is very invasive and should be avoided. If you must grow it, keep it in a container.
Some herbs can be confused with similar-looking plants - an example being dandelion (Taraxacum officinale). Some people use the young leaves in salads, and the plant is also claimed to have medicinal properties. However, numerous other plants whose properties are unknown have similar leaves and flowers.
Gotu kola (Centella asiatica), the so-called arthritis herb, can easily be confused with invasive hydrocotyle or pennywort (Hydrocotyle ranunculoides and H. verticillata) because of the similar leaf shape and habit and a preference for wetland locations. As the properties of hydrocotyle are unknown, eating it would be risky.
Implications of harvesting from beyond the garden
In the above example of gotu kola, even if you are confident about being able to identify it correctly, do not collect it from natural wetland habitats because this practice depletes wild populations and could threaten the plantâ€™s survival. Gotu kola is a native plant that is protected by Western Australiaâ€™s wildlife protection laws.
Elsewhere in the world, many plants are under threat due to over-harvesting by both commercial companies and private individuals that manufacture and supply herbal products, notably essential oils. It is ironic that the â€śgreenâ€ť appeal of supposedly healthy alternatives is putting natural ecosystems in danger.
On the domestic front, collecting herbs from beyond the garden has its hazards. For example, a dandelion picked from the road verge has been exposed to traffic fumes and, possibly, pesticides during council spraying programs. Similarly, watercress (Rorippa nasturtium-aquaticum) collected from a ditch on public land could be growing in polluted water.
Herbal ornamentals and garden curiosities
Rosemary (Rosmarinus officinalis) is one culinary herb that has long been used as an ornamental. Parsley, too, is finding its way into some flower borders, as it is a nice green edging plant in the short term.
Of course, even people who do not have the slightest interest in self-medication may still choose to grow medicinal herbs simply for their aesthetic appeal - which is fine provided those herbs have no invasive tendencies. Some medicinal herbs have attractive flowers - for example, echinacea and marigold (Calendula officinalis).
Lesser known or even obscure herbs may attract plant enthusiasts who are keen to acquire new curiosities - but if you are this kind of enthusiast, please remember that certain herbs may be highly invasive and must not be brought into Western Australia.
An example is purple flower devilâ€™s claw (Proboscidea louisianica), which has a woody fruit with claws that could injure livestock. This herb is a declared plant in Western Australia. Do not grow it.
Also, do not order any seeds from overseas, or seeds and plants from other States in Australia, or the Internet, until you have checked with quarantine (Tel. 9334 1800). This will protect you from inadvertently bringing in weedy species or something potentially infected with pests or diseases.
If you have unwanted herbs in your garden, pull them out, put them into plastic bags, tie and bin them. Alternatively, if you are taking your garden rubbish to a waste tip, transport it in a covered trailer so that seeds and cuttings cannot fall onto the roadside and grow. Never dump unwanted herbs - or any other plant material - in bushland.
Page updated: October 2008
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