The Weeder’s Digest

Identifying and Enjoying Edible Weeds

By Gail Harland, Debs Cook, Dave Hamilton, Gary K Smith/Alamy, iStock, Gary K. Smith/Alamy FloraImages/Alamy

Green Books Ltd

Copyright © 2012 Gail Harland
All rights reserved.
ISBN: 978-1-900322-99-7

Contents

Introduction,
PART 1 Know your weeds,
1 The good, the bad and the ugly: Characteristics of weeds,
2 Achievable weed control: A question of balance,
3 A case of mistaken identity: Poisonous plants,
PART 2 A bouquet of weeds,
Glossary,
Notes,
Bibliography,
Index,

CHAPTER 1

The good, the bad and the ugly

Characteristics of weeds

A large number of plants can be classed as weeds, although, as a proportion of known plants, surprisingly few are really troublesome. Any species may be considered a weed in a garden if it grows to dominate other plants, but most weeds have a number of common features. They are usually fast-growing and able to out-compete other plants for food, light and water. They are tolerant of a variety of soils and situations. They spread readily by seed or vegetative growth. Some weeds may show resistance to herbicides and be very difficult to control.

Weeds will vary depending on the conditions in which they grow. A weed can be relatively innocuous in one garden but run rampant in another where the conditions are more to its liking. Some may be found in just a few parts of the country, whereas others, such as groundsel, may occur in virtually every garden.

Of course, as well as the weeds themselves, gardeners vary. Some people aim for a perfectly kept garden and show zero tolerance for any weeds. Other gardeners may not be so particular and can enjoy the shaggy golden flowers of dandelions without feeling the urge to leap up and grub them out. Much depends on the style of garden; weeds may be accepted in a casual orchard setting that would not be tolerated in a formal flowerbed close to the house.

Perhaps the important thing is to get to know your weeds and their characteristics and to understand which of them can be tolerated, used or even enjoyed, and which are best kept firmly under control.

Annual weeds

Annual weeds are those such as shepherd’s purse and chickweed, which complete their entire lifecycle within one year.

Some, such as hairy bittercress, are termed ephemerals, as they have a particularly short lifecycle with several generations produced in one season. Thale cress (Arabidopsis thaliand) has a lifespan of just four to six weeks. Its short lifecycle has proved of value in the study of molecular biology, and this species has even been grown in a shoebox-sized mini-glasshouse on the International Space Station. While its life may be short, it is certainly productive, and a single plant of thale cress can produce nearly 3,000 seeds.

The old adage ‘One year’s seeding means seven years’ weeding’ is certainly true in the case of annual weeds, and to control them it is important that they are removed before they have a chance to set seed. Many produce small light seeds that are easily distributed on the wind and are quick to germinate. The seeds can, however, lie dormant in the soil for many years until they are brought to the surface, when they will germinate and begin to grow rapidly. Annual weeds are found most often in places which are regularly cultivated or disturbed, for example arable land and in vegetable plots or on allotments. They grow rapidly and can compete with and smother slower-growing plants.

Perennial weeds

Perennial weeds such as spear thistles are able to survive from year to year. Herbaceous perennials usually die down to ground level in the winter and survive as fleshy rootstocks or rhizomes, for example docks and stinging nettles. The roots of some species can go down to great depths in the soil, making them extremely difficult to dig out. Horsetail roots have been found at depths of 2m (6’6″). Deep-rooted perennials will usually regrow if the top is pulled up or burned off. There are also many woody-stemmed weeds, including brambles, elder and ivy. Tree seedlings such as ash and sycamore can be a particular problem in many gardens.

Perennial weeds spread by means of seed, or vegetatively, often by creeping rootstocks. Perennial weeds are often more difficult to control than annuals, as hoeing and digging may break the roots and rhizomes into pieces which are often able to regrow into individual plants. The bulbs of wild garlic may be distributed by digging or even by burrowing mammals such as rabbits. Woody weeds with thorns such as brambles and roses can be painful to deal with, as the long spiny stems need to be cut down before you can attempt to dig out the roots.

Spreading the seed

Weeds are notorious for the prolific quantity of seeds they can produce. A single plant of shepherd’s purse can produce 150,000 seeds and common purslane can produce a staggering 1,800,000 seeds per plant. The majority of these seeds lose viability quite quickly, but some can remain viable in the soil for 40 years or more, awaiting suitable conditions for germination. It is not just the quantity of seeds produced, but also the ingenious ways in which they are dispersed that enables weeds to be so successful.

Wind

The seeds of many plants have elegant silky plumes that enable them to be carried for long distances on currents of air. The dandelion clock with its seeds surmounted by silvery hairs is the classic example of this. Other examples include thistledown, bulrushes and the fluffy seeds of the willowherbs. Tree seeds often have stiff wings enabling them to spin as they are carried along by the wind. The seeds of the ash tree, known as keys, have just one wing, whereas sycamore seeds are dispersed as two-winged pods that later split into two separate portions to release the seeds. The seeds of weeds such as poppies and corncockle develop in pods like little pepper pots and are shaken out by the wind.

Mechanical

Oxalis species, hairy bittercress and many legume plants have seedpods in which great pressure builds up as the pod ripens. It then takes just the slightest touch for the pressure to release and the contents to be flung over a wide area. If you walk past heath land gorse bushes on a hot sunny day, you can often hear the explosions as the seedpods burst open. The seeds of Himalayan balsam can be dispersed 7m (23′) away from the parent plant by this method.

Water

Obviously, aquatic plants will depend on water to carry their seeds downstream to colonise new areas, but many land plants too will use water. Himalayan balsam seeds that land in water after being ejected are carried away, buoyed by the corky seed coat. The jimsonweed (Datura stramonium) has flat, corky seeds that float easily.

They can be carried downstream and deposited on the riverbanks, where they will germinate. The giant hog-weed can also spread along riverbanks by virtue of its corky seeds.

Birds

Many birds will cheerfully gorge themselves on the fruit of plants such as brambles and elder. They can then fly far from the parent plant and deposit the seeds complete with a neat dab of fertiliser to ensure a good start in life for the new seedling. Some birds such as jays will deliberately cache seeds, for example the nutrient-rich acorns of oak trees, for eating later. Inevitably not all these saved seeds are retrieved and many may well live to germinate in spring.

Hitchhiking

The burrs of weeds such as burdock and goose grass are perfectly designed to catch on the fur of passing animals, hitching a lift to pastures new. They will be spread just as efficiently by hooking themselves to the gardener’s trouser legs, so it is worth paying attention if you have been working where these plants grow to ensure that you do not inadvertently spread the seeds around the garden. Seeds of plantains, flaxes (Linum spp.) and many mustard species become viscid when wet and will stick to the soles of shoes or the tyres of cars and so can be spread in this way. Weed seeds may be carried right across the world if they occur as contaminants in transported crop seeds, or as stowaways in trucks and ships.

The root of the problem

Plants with a large storage root like carrots and dandelions have a taproot system consisting of one large straight root from which lateral roots may radiate. The depth of the primary root frequently exceeds the height of the plant above ground. Weeds such as grasses often build up a dense fibrous root system that consists of several branching main roots which form a mass of intermeshed lateral roots. One study found that a single rye plant 50cm (1’8″) tall with 80 shoot branches had a root system that equated to 210m2 (2,260 sq ft) of surface area compared with just 4.5m2 (48 sq ft) for the above-ground part of the plant.

Perennial weeds often have rhizomes, underground stems that usually grow through the soil close to the surface. These have buds along their length that can develop into new shoots. Weeds that spread by this method include nettles and ground elder. Plants such as silverweed spread by stolons or runners. These are a type of stem that creeps horizontally along or under the ground and forms new plants at the tip. They are similar to rhizomes, but sprout from an existing stem. Stolons typically die away once the new plant has grown, whereas rhizomes will live as long as the parent plant.

Roots that develop from organs other than the main root system are known as adventitious roots. They generally develop at the nodes of the stem. Brambles readily form roots at the tips of canes if they are in contact with moist soil. Particularly long stems that are growing along the ground may root at several places along their length, resulting in a dense thicket of plants growing around the parent plant.

Some plants, including many in the pea (legume) family such as black medick, are capable of changing atmospheric nitrogen gas into ammonia, a form of nitrogen that is usable by plants. This is done within the root nodules in which the bacterium Rhizobium lives. The bacterium actually fixes the nitrogen, in a relationship that is beneficial to the plant. When the plant dies the nitrogen is available for use by any subsequent crops grown in that soil.

Why bother weeding?

It is not only lazy gardeners who sometimes wonder why they bother doing any weeding. Watching the bees bumbling drunkenly from one clover flower to the next, or a red admiral butterfly carefully laying its eggs on a nettle leaf, many of us may ask ourselves why we persist in struggling against weeds. Might it not be better to leave things to Mother Nature? There are, though, sound horticultural reasons why gardeners are exhorted to be vigilant about weed control.

Competition with crops

Successful weeds are usually highly competitive, and in the race for resources our chosen garden plants or crops are often left standing. Fast-growing species such as chickweed quickly cover the soil and use up water and nutrients, while sprawling over slower-growing plants, and denying them access to light. Russian vine was introduced to many gardens as a fast-growing climbing plant, appreciated for its lacy flowers. However, it grows so quickly that it soon smothers any plant in its path. Studies looking at the effect of the weed fat hen on the growth of crops found that it was a problem in many major crops. It can grow quite tall and shade the crop plants, so even a low population can have a significant effect. When well established it could reduce yields of sugar beet by up to 48 per cent.

Some weeds may show allelopathic properties. This occurs when the weed releases chemicals which can have an effect on the growth of other plants in the environment. Much of the work regarding allelopathy has been done in the laboratory rather than the field, but it certainly seems that plants such as mugwort become such successful weeds not just by out-competing other vegetation but by actually poisoning that are detrimental to the growth of other plants. Jack-by-the-hedge releases chemicals known as isothiocyanates, which inhibit seed germination and plant growth. This effect seems to have contributed to the dense stands of this weed that have developed in many parts of the USA.

The concept of allelopathy is not new. As early as 300 BC the Greek philosopher Theophrastus observed that the chickpea reduced nearby weed growth. Research is currently focused on developing crop plants that will themselves have allelopathic properties on weed growth, so that there will be less need for herbicides to control weeds in the crop.

Harbouring pests and diseases

Weeds can act as harbours for many pests and diseases. The pests can include rats and mice, who may find weed-infested areas of the garden to be ideal places to nest. Smaller pests include many insects that will feed and breed on weeds and potentially spread to ornamental or crop plants.

Weeds in the Polygonaceae family such as docks, sorrels and knotgrass (Polygonum auiculare) act as hosts to dock bugs. Adult bugs are seen in late spring and again from August to October. They are a brownish colour with a broad, flattened body up to 12mm (½”) long and large antennae. They do not cause a problem on the dock plants, but in late summer they feed on raspberry and blackberry fruit. If disturbed they can release a pungent scent from glands on their abdomen which will taint the fruit.

Most insects are specialist feeders, with a limited range of food plants, so weeds that are closely related to crops are more likely to host the pests that attack those crops. Insects, particularly sap-sucking ones such as aphids, can also act as vectors for disease.

Dense weed growth can raise the humidity in a microclimate, increasing the incidence of many fungal diseases such as powdery mildew. Powdery mildew diseases are caused by many different species of fungi in the order Erysiphales. They infect a wide variety of plants including cucumbers, courgettes and many ornamentals such as phlox. Infected plants show white powdery spots on the leaves and stems, and a general weakening of growth.

Harmful weeds

Many weeds are potentially harmful to the health of humans and to their domestic animals or pets.

Hogweed Hogweed (Heracleum sphondylium) and its big cousin giant hogweed (H. mantegazzianum) are both actually edible if the young spears are harvested when the flowers are forming within the leaf sheaths. Cooked as broccoli, this is a succulent vegetable with a sweet, aromatic flavour. The plants are often recommended as a suitable forager’s food. However, they can also cause the unpleasant condition known as meadow dermatitis, which is characterised by reddening of the skin, swelling and the formation of large lesions and blisters. Reddening usually takes place 24 hours after contact with the plants and blistering up to 3 days later.

In 2010 a 10-year-old Irish boy had to receive a skin graft after developing severe burns from giant hogweed. The burning occurs because the sap and bristles on the stem and leaves contain furocoumarins, which make the skin hypersensitive to bright sunlight. Strong illumination and high humidity will intensify the reaction. The fruit and roots are also phototoxic.

The dermatitis often occurs after strimming areas of rough grass in which hogweed is present, as the strimmer sprays the sap out most effectively. If you are exposed to hogweeds, you should wash and cover your skin immediately to prevent further exposure to sunlight.

Ragwort The toxic properties of the common weed ragwort (Senecio jacobaea) have been widely publicised. It contains pyrrolizidine alkaloids which cause serious liver damage. Ragwort is one of the commonest causes of plant poisoning in farm animals. Cattle and horses are most frequently affected, but prolonged feeding by sheep, goats, pigs and poultry is also harmful. The animals may not show symptoms until they have been eating ragwort for several weeks, but large amounts eaten at one time will cause the more rapid onset of symptoms. First signs are usually abdominal pain and diarrhoea, progressing to deterioration in general condition, photosensitisation, agitation, loss of coordination and death. There is some concern in the USA about the toxic alkaloids being found in honey in areas where ragwort is the dominant flower.

Ragwort is a member of the daisy family (Asteraceae). It has deep-green, lobed leaves and large numbers of small daisy-like flowers of a deepyellow colour, on plants growing 40-90cm (l’4″-3′) tall. The plant has an unpleasant smell. Ragwort has a bitter taste so most horses will avoid it unless there is nothing else available to eat. However, occasionally individual animals seem to develop a taste for it and can eat it in quite large quantities. When ragwort is dried in hay the bitterness disappears but the toxic properties remain and it can then be a serious problem as the animals will eat it. Groundsel (S. vulgaris) contains alkaloids similar to those in ragwort but it does not usually grow in sufficient quantities to cause poisoning.

Other toxic plants Of course, many plants contain potentially deadly toxins. Foxgloves, hemlock, yew and aconites are well-known examples. Even apparently innocuous plants such as the attractive rhododendron shrubs that have naturalised themselves in many places can produce grayanotoxins, which dramatically lower blood pressure and can even cause death. An account in Greece from the fourth century BC relates how 10,000 soldiers were poisoned by the honey of honeysuckle azalea (Rhododendron luteum). A more recent case occurred in Scotland when a man licked some drops of rhododendron nectar from his hand and suffered a temporary loss of coordination and inability to stand.

(Continues…)Excerpted from The Weeder’s Digest by Gail Harland, Debs Cook, Dave Hamilton, Gary K Smith/Alamy, iStock, Gary K. Smith/Alamy FloraImages/Alamy. Copyright © 2012 Gail Harland. Excerpted by permission of Green Books Ltd.
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