Gardening, Jorge, Weeding

To many gardeners, weeds are a recurrent annoyance that you have to put up with as a fact of life. They can interrupt your otherwise perfect lawn or stifle your crop yields, and at worst cost thousands of pounds in damage as in the case of the Japanese knotweed. The effects of weeds are extremely costly, and it has been estimated that up to 10% of agricultural production may be lost because of them. But, weeds play a key role in transforming inhospitable environments into new habitats, and without them we would not exist today.

Weeds are good for the ecosystem

Weeds are important as they play a key role in transforming barren earth into rich fertile soils. They are, in effect, pioneers as the first plants to colonise a piece of land and improve its soil for the development of more complex ecosystems. They do this in a number of ways.

Weeds act to shield the soil from the sun, protecting both insects and microscopic organisms from sunlight. Their roots stabilise the soil, creating a secure environment for life, while their stems trap organic matter, which breaks down in the soil and provides sustenance for insects. Weeds with long roots draw up nutrients from deep in the ground, improving the quality of the surface soil. When they finally die, they decompose into humus which increases the soil’s moisture and nutrient retention, and well its bulk density, which is important in the early stages of soil development.

Curled Dock with its deep taproot draws up nutrients to the surface soil. Picture Credit:  Oliver Pichard (2007) licensed under CC BY-SA 3.0.

Back hundreds of millions of years ago, the Earth was very different, as a barren rock with water running over the surface with no defined course. Key to transforming the Earth were plants that broke down rock into minerals and soil, which it then held in place with its roots. This led to the development of river banks that channeled water in a regular fashion. Periodically, such rivers would flood, depositing sediment over large areas, which allowed trees to take hold. Such larger plant life would produce even more debris that would block up rivers, causing more flooding, a process that would lead to the emergence of larger complex ecosystems.

The predecessors to the plants that we consider weeds today played a key part in all this as early pioneers that ensured soil stability in such flooded areas. Important to this were rhizomes that allowed plants to cope with severe disruptions in their environments. Rhizomes are branching stems that grow horizontally, often through the soil, and are the feature that makes weeds so durable, as even if you destroy a plant’s matter above ground any surviving rhizome in the soil will lead to its reemergence. Not only does the rhizome store energy, allowing a plant to reemerge in favourable conditions, the stems allow the plant to propagate vegetatively,  producing a clonal plant.

An artist’s (Édouard Riou) impression of early Devonian land flora.

One early example of a plant that helped stabilise the earth’s environment was the Drepanophycus from the Devonian Period, which was unearthed by a team from Peking University. It was discovered preserved in paleosols – fossil soils – that within were multiple sequences of sediment formed by river channels, which were periodically wiped out by floodplains. The plant grew continuously due to its rhizomes and trapped sediment, enabling stable soils to develop. And after the floods, the plant would reemerge, growing through the newly deposited layers of sediment. The team calculated that the plant had a modest, but significant role, in reducing soil erosion. It is believed to have carried out this function for centuries.

Weeds constitute an interesting case study in evolution and humanity’s effects on the environment

Today weeds constitute a fascinating area of study due to their phenotypic plasticity, or simply put, their ability to change in response to changes in their environment. An example of phenotypic plasticity may be a plant’s ability to utilise more or less water (in photosynthesis) depending on its availability. Phenotypic plasticity is especially important for plants that do not have the ability to change their environment (as in the case of many animals, such as humans), and weeds are especially adaptive as agricultural practices make it necessary to be highly responsive if they are to survive.

Weeds evolve quickly in three principal ways: through adapting to continuous habitat disturbance, emerging in part from agricultural practices; through reproducing with different cultivars (groupings of plants selected for certain characteristics) as to produce hybrids; and finally through returning to natural seed dispersal methods when certain domesticates (plants dependant on humans for survival) are abandoned. This has led to the survival of certain species that are extremely difficult to control as they have developed such traits as early germination, rapid growth from seedling to sexual maturity, and the ability to reproduce both sexually and asexually. Fascinatingly, a 2013 study carried out by Fudan University of Shanghai found that if genetically modified crops did crossbreed with their weedy cousins, the resultant weeds would have higher rates of photosynthesis, more stems and flowers, and significantly more seeds. So, in the future, weeds may become even more troublesome than they are now.

Centuries of grazing has altered the landscape, benefiting plants that can’t be consumed by livestock.

As such, the battle between farmers and weeds constitutes an interesting case study of evolution in action and the selection effect humans exert on plants. There are many examples of the latter. For example, tilling tends to favour annuals at the expense of perennials, while no till systems benefit perennials. Frequent mowing, on the other hand, tends to benefit weeds that grow horizontally. The grazing of livestock has led to an increase in noxious thistles and other inedible species on the rangeland. In some cases, weeds have even begun to replicate crops in their appearance and life cycle as in the case of barnyardgrass growing with rice.

Weeds perform an important signalling function

Weeds can tell you a lot about your garden, providing information about what is best to grow. If your weeds multiple rapidly it is likely that your soil is extremely fertile, and that you do not need fertiliser. If not, it may be wise to start growing forerunners such as onions before moving onto more difficult crops. If the amount of weeds is diverse, it is likely that you can grow a wide range of plants in your garden. If not, it will be worthwhile to ascertain the soil type. And weeds can do this too. Very acidic soil will produce sorrel and plantain but no charlock or poppy, while chickweeds is sign of neutral pH. High levels of nitrogen can be ascertained by nettles, ground elder, fat hen and chickweed. Compacted soil is noticeable for silverweed and greater plantain, while creeping buttercup, horsetail and silverweed may indicate wet soil with poor drainage.

Weeds constitute a good source of nutrients

Dandelion leaves are high in vitamin a and k and can be useful addition to a balanced diet.

Many weeds are edible and good for you. They are also effectively free and environmentally friendly. In the UK, nettle soup comes to mind as one famous example. Back in the Middle Ages, ground elder was grown as a crop and was believed to cure gout – hence its alternative name goutweed. It possesses a nutty flavour and can be added to salad. Many health blogs recommend dandelion as a superfood, which can be found everywhere. Sorrel and horseradish can both be made into sauce and the latter is often used with beef. There are many great blogs dedicated to eating and cooking wild food. Why not check them out for yourself?

A concluding thought

Perhaps, our obsession with weeds tell us more about ourselves than we think. Why are we pursuing them with such vigour? Instead of hastily striving for a perfect world without weeds, perhaps we should examine why they are there in the first place. After all, a weed is a plant whose virtues have yet to be discovered (Emerson, apparently).

Jorge at PrimroseJorge works in the Primrose marketing team. He is an avid reader, although struggles to stick to one topic!

His ideal afternoon would involve a long walk, before settling down for scones.

Jorge is a journeyman gardener with experience in growing crops.

See all of Jorge’s posts.

Jorge, Plants

Green plants appear green due to a pigment called chlorophyll that primarily absorbs blue and red wavelengths of the visible light spectrum, but reflects a portion of green wavelengths. This green light enters our eyes and hits the light-sensitive retinas, in which there are cone cells, that once stimulated, sends a signal to our brain that interprets the information, giving the colour green. Therefore it can be stated that the colours of an object is dependent on what colours are reflected (or transmitted) back to our eyes. (Technically speaking,  visible wavelengths have no colour. Colour is created in the brain.)

Most humans are trichromats, and possess three types of cone cells sensitive to red, green and blue light, named L M and S respectively. Each cone allows us to distinguish around a hundred shades, so the total number of combinations is at least a million. Colour is determined by our brains that interpret the different ratios of these three colours.

The visible light spectrum ranges from approximately 400nm to about 700nm. Our brain attaches different colours to different wavelengths with blue at about 475nm, green at about 510nm and red at about 650nm. Picture Credit: Vanessaezekowitz (2007) licensed under CC BY-SA 3.0.

Not all humans, or all animals, perceive colour in the same way. Dichromats, such as dogs, possess two types of cone cells and can distinguish blue and yellow, but not red and green. Their vision is similar to some colour blind humans, who only have two working cone cells due to either an absence or a malfunction of a third type of cone cell. Not all colour blind humans are the same as they can have different combinations of working cone cells (or none at all), and thus are unable to see different colours, resulting in different colour spectrums.

Some animals are tetrachromatic and able to distinguish to four primary wavelengths of light. Birds, for example, are even able to view ultraviolet light, which is beyond the visible light spectrum. (Interestingly, humans with Aphakia can also view ultraviolet as their lens has been surgically removed. For the rest of us, our lens blocks this light.)

Some women are tetrachromatic as they possess four types of cone cells, which allows them to see a hundred million colours. The extra cone cell has its origin in their fathers’ colour blindness, who possess two working cone cells and one mutant one. This mutant one is passed on to the daughter, who then has four cone cells. It is probable that tetrachromats have to train themselves to see such an array of colours, as the natural world will not have such a diversity of colours for the brain to learn to use the fourth cone. As such, it is likely that most will go through life without recognising their potential.

The absorption spectrum of a bird’s (Estrildid finches) four cone cells.

So tetrachromats, both human and non-human, can distinguish many more hues of green than  the rest of us, and plantlife may appear very different. For animals like birds this may be very useful for distinguishing between plants to find sources of food or shelter.  For the rest of us, our trichromatic vision proves very useful in allowing us to quickly identify between opportunities for profit and sources of danger, such as when fruits are ripe.

Plants need to absorb light in order to carry out photosynthesis to produce glucose, which can be used for metabolism and growth, or stored as starch. Photosynthesis is a chemical reaction that inputs sunlight, water and carbon dioxide and outputs glucose and oxygen. It is a two step process, comprised of light-dependent and light independent reactions. In the former sunlight plays a key role by providing the chlorophyll with energy to kickstart the complicated chemical reaction.

In green plants, there are two types of chlorophyll: chlorophyll a and chlorophyll b that both absorb different spectrums of light. They both complement each other with a absorbing more red light and b absorbing more blue, and this allows the plants to fulfil its energy requirements. As you can see in the graph below, chlorophyll still absorbs green light but not to the same extent as they do red and blue.

Picture Credit: Daniele Pugliesi (2008) modified by M0tty licensed under CC BY-SA 3.0.

However, this is not the full story. The above graph represents the absorption spectra of extracted chlorophyll molecules. As part of a plant, chlorophyll never exist alone but are bound to molecules that influence what it absorbs, and as such plants absorb about 70% of green light.

There are other pigments (accessory pigments) inside green plants that play a role in photosynthesis such as carotenoids. They primarily absorb green and blue, but reflect yellow, orange and red. It is these pigments that give many plants’ leaves their autumnal colours, and signal the presence of ripe fruit, once the amount chlorophyll is reduced. These accessory pigments are useful as they allow the plant to capture more of the sun’s energy by broadening its absorption spectrum.

So, what about plants that aren’t green? While all plants that photosynthesise contain chlorophyll a, they can contain many different types of accessory pigments, giving them different colours. For example, many reddish-purple plants contain the pigment anthocyanin in such abundance that acts to mask the green chlorophyll pigments.

So, why do plants use red and blue light more so than green? And why do they not absorb all visible light (and henceforth appear black)?

It is believed that today’s plants evolved from a common ancestor (green algae) that used chlorophyll to photosynthesise. Why no alternative dominant pigment emerged is an unanswered question, although many hypotheses have been proposed. Evolution is a product of multiple processes such as random mutation, random selection and natural selection, and henceforth plants can’t design or choose the best pigment to use. It is therefore probable that once chlorophyll proved successful no new alternative dominant pigment emerged, thus enabling green plants to dominate the landscape. Although, there is a possibility that (primarily) utilising a narrow band of wavelengths (red and blue) for photosynthesis is mechanically superior, and this allowed early organisms to outcompete other lifeforms.

For more discussion on why plants use chlorophyll, and are henceforth green, can be found here, here and here.

Why do plants use the visible light spectrum for photosynthesis?

In general, plants only absorb trivial amounts of light outside of the the visible light spectrum. This is because the sun produces the most light in the visible light spectrum, and chlorophyll have evolved to utilise it. (If you look at the graph above, chlorophyll a’s absorption spectrum is almost exclusively confined within the visible light spectrum.) There are other mechanical reasons for this. Visible light is perfect as it provides just enough energy without causing damage to the plants’ cells. By contrast, ultraviolet is damaging and infrared contains insufficient energy. In addition, a lot of ultraviolet light is blocked by the ozone layer.

Jorge at PrimroseJorge works in the Primrose marketing team. He is an avid reader, although struggles to stick to one topic!

His ideal afternoon would involve a long walk, before settling down for scones.

Jorge is a journeyman gardener with experience in growing crops.

See all of Jorge’s posts.

Composting, Gardening, Jorge, Plants

mycorrhizal fungi
A mycorrhizal fungus as viewed under the microscope. Picture credit: Dr. David Midgley (2007) licensed under CC BY-SA 2.5.

Mycorrhizal fungi rootgrow has become a common feature of garden centres of late, and has been advertised as a product that can greatly boost your plant’s health. But does it really work? And when should I apply it?  Before delving into such questions, it would be worthwhile to explain what are mycorrhizas.

What are Mycorrhizae?

The etymology of Mycorrhiza comes from the Greek mykos “fungus” and riza “root”. And this is precisely what mycorrhizae is, a symbiotic relationship between fungi and plants. It occurs in nearly all plant life on land and is thus suspected of being one of the key factors that allowed plants to colonise the land.

The relationship is symbiotic as the fungi and plant provide one another with nutrients that each are maladapted to garner independently. It has its origin in the fact each are different types of organisms, with fungi being heterotrophic and plants autotrophic. Heterotrophs, such as humans, absorb their nutrients from organic sources, but can’t produce energy from inorganic sources. Autotrophs, on the other hand, can produce energy from inorganic sources such as sunlight. Plants do this through the process of photosynthesis that produces carbohydrates. As autotrophs, plants also find it difficult to absorb essential nutrients such as nitrogen and phosphorus.

endomycorrhizae
Mycorrhizal fungi located inside a flax root’s cortical cells as viewed under the microscope.

And this is where the fungi come in. The fungi that can easily absorb such nutrients interacts with the plant’s root system, which the plant willingly allows, providing such nutrients in return for the carbohydrates that itself cannot produce.  It does this through expanding its roots’ surface area that can absorb nutrients and water. They also provide the additional benefit of increasing a plant’s resistance to pathogens, preventing root disease.

As a side-note, the mycorrhizas were once divided into broad groupings, the ecto (outside) and endo (inside) varieties, with the former (usually) coating the root cells and the latter intermeshing into the plant root cells; although today they have been divided into new sub-categories or superseded with new typings. The endo varieties are difficult to spot, while the ecto varieties presence may be hinted at with the appearance of toadstools, or coated, oddly-branched roots.

The Leccinum aurantiacum – an ecto variety of mycorrhizal fungus. Picture Credit: Tomas Čekanavičius (2006)  licensed under CC BY-SA 2.5.

Do I Need to use Mycorrhizal Rootgrow?

It is suspected that neither fungi nor plants could survive in many situations without such a relationship. Mycorrhizas is fairly ubiquitous throughout the soil, and can infect a wide range of plants, so it is highly probable that suitable plants will become infected in their lifetime. There may be some exceptions to this, such as heavily cultivated soil and isolated rocky outcrops, but more on this later.

Scientifically, there is little evidence supporting the use of mycorrhiza rootgrow. The British Standards Institution, which produce technical standards on an array of products, does not recommend using the rootgrow for planting trees as a matter of routine. At Texas A & M University, a team grew plants in soils with and without mycorrhizas and found that the infected plants grew slightly better at the planting-out stage, although any advantage disappeared completely after two seasons in the ground. This was because all the plants ended up infected with mycorrhizas anyway. Finally, a test by Which? Magazine found that the potting compost brand that contained mycorrhizas performed poorly, although such an outcome may be down to other factors.

So are mycorrhizas products any good at all? In all probability mycorrhizas products will be unlikely to confer any long term benefits to your plants, unless you have good reason to suspect that your soil is deficient in mycorrhizas. (It is important to check that your plant can benefit from mycorrhizas in the first place.)

Heavy use of phosphorus in agriculture reduces the incidence of mycorrhiza in the soil. The element is also used to ignite matches.

As already stated, heavily cultivated ground may reduce the occurrence of mycorrhizas. This is because fungicide is naturally destructive to fungi, although, interestingly, it is phosphorus-rich soil that is especially detrimental to mycorrhizas. Mycorrhizas usually function to gather this rare resource for plants, but an abundance of it, usually created by fertiliser, actually suppresses it. Why this is the case is unclear, although it can be in a sense expected, as messing with the ecosystem can have untold effects. Sadly, in this case using mycorrhizas rootgrow is unlikely to have any effect, as if the soil is not conducive to mycorrhizas, the best option will be to stop using phosphorus rich fertilizers, and wait for the mycorrhizas to return naturally.

There may be reason to use mycorrhizas in some cases, perhaps for isolated plants, and plants that are indoors (although such cases are unlikely to be common).

Jorge at PrimroseJorge works in the Primrose marketing team. He is an avid reader, although struggles to stick to one topic!

His ideal afternoon would involve a long walk, before settling down for scones.

Jorge is a journeyman gardener with experience in growing crops.

See all of Jorge’s posts.

Hedging, Jorge, Plants

alternatives to buxus

Forgoing box is a real shame as it possesses all the characteristics required for low maintenance natural hedging. It responds well to clipping, and is slow growing, often needing to be cut only once a year, with growth usually between 10 and 15cm. It is also frost resistant and native to the UK, being cultivated since at least Roman times. Sadly, due to the current box blight epidemic, box is no longer the premium option, as the disease can destroy years of work to which the gardener can do little to stop. However, using other plants can be seen as a great opportunity to experiment, which makes gardening so enjoyable in the first place. There are so many underappreciated alternatives that can produce stunning delineated gardens.

Obviously, no plant will be exactly like box and the shape (and colour) of your hedge could be very different. If you wish for a substitute for box, however, Privet (Ligustrum ovalifolium) is a highly popular choice that is extremely hardy and small leaved, although sadly fast growing; henceforth, it will need to be trimmed multiple times in the summer to encourage dense growth. As it is only semi-evergreen, there is also the possibility of the plant shedding its leaves in extreme bouts of cold. Another possibility is switching to artificial topiary that is visually identical to box, and virtually indestructible, although its shape is limited to the manufacturer’s designs.

alternatives to box

Other worthy alternatives include the Griselinia littoralis, Euonymus japonicus and Elaeagnus ebbingei. The Griselinia is notable for its soft glossy leaves, average growth rate and responsiveness to clipping. The Euonymus is usually two-tone with cream bordering the edges of its otherwise green leaves, although it can variegate greatly in full sunlight. The plant is hardy and suitable for nearly all soil types, although will need maintenance to ensure denseness. The Elaeagnus is a great alternative as it is dense, hardy and responsive to clipping. It is also fragrant in the autumn with the emergence of white flowers.

One of the best species of natural hedging has to be the Taxus baccata, commonly known as the English Yew or Common Yew; it is very hardy, average growing, dense and great for birds, which love its berries. For more colourful alternatives, lavender (Lavandula angustifolia) and sometimes heather, can be grown into hedges. Key for lavender is to cut it before it flowers, or otherwise it will lose its shape. It is great for wildlife, fragrant and evergreen.

common yew

Due to the box blight epidemic, the RHS Garden Wisley are currently trailing 25 alternatives to boxwood. The varieties that have performed well include such plants as the Kilworth Cream (Podocapus nivalis), Sunshine (Ligustrum sinense) and Tom Thumb (Pittosporum Tenuifolium). The team has found that the Podocapus versatile, and responsive to clipping; the plant itself can be described as extremely small leaved, and darker in colour than box. The Ligustrum is slow growing with vibrant yellow leaves, the Pittosporum purple and compact. Also of interest is how the Pittosporum is a source of food for animals in its native New Zealand and is thus hardy and responsive to clipping.

Do you have any experience growing hedges? We’d love to hear from you. Post in the comments below!

Jorge at PrimroseJorge works in the Primrose marketing team. He is an avid reader, although struggles to stick to one topic!

His ideal afternoon would involve a long walk, before settling down for scones.

Jorge is a journeyman gardener with experience in growing crops.

See all of Jorge’s posts.

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