In this post in this series on weed control, I cover not just how weeds survive, but how weeds thrive in the battleground of our gardens.
Ultimately, I’ll look in detail at some of the ways we can prevent weeds from causing us problems and at how we can deal with problem weeds when they do get through our defences.
But before we get to those topics, it’s important that we take a close look at some of the tactics that weeds themselves use to try to defeat us.
This is essential because weeds have evolved and adapted to be amongst the most efficient, successful and tenacious species in the plant kingdom and it is only really by understanding the way the weeds survive and thrive that we can hope to succeed in preventing them from doing just that.
Like other plants, weeds can be classified according to their life-cycle. This is important information for us when we want to cultivate particular plants, but it is important for us also when it comes to dealing with the plants that we very much do not want to cultivate. It is useful when we look at ways to deal with weeds, to classify them according to whether they are annual or perennial plants.
Perennial plants are plants that live for more than 2 years. In most cases, perennials can exist for many years. Plants that you may grow in your garden such as Echinacea, foxgloves, clematis or asters are all perennial plants. Usually, woody plants such as trees and shrubs are excluded from the definition of perennial plants, although they strictly qualify because of their longevity. Sometimes bulbs are excluded too, but for present purposes we will include bulbs within the perennial plant ambit, since there are many troublesome weeds with bulbous energy storage systems.
Annuals are plants that complete their entire life-cycle in one year. They germinate from seed, grow, flower and set seed all in one year, thus passing on the genetic profile of the plant to the next generation within that short period.
There are some plants which complete their life cycle in a period of 2 years. These are called biennials. There are even some plants that complete more than once life-cycle in a single year. These are called ephemerals. For our purposes here, we can treat biennials and ephemeral in the same way as annuals, since they reproduce in exactly the same way.
The seed is a pretty miraculous product of evolution. Each seed that a plant produces contains within it all the genetic material that is needed to produce a new plant. But, what makes this reproductive technique really effective, is that once produced by the mother plant, most seeds can remain dormant, but viable, until conditions arise which are suitable for the growth of the new plant.
Annual weeds take supreme advantage of the long-term viability of seeds. The seeds of many weeds can remain dormant in the soil for years – it is said that poppy seeds can retain their viability for up to 100 years. This means that at any one time there is a huge store of weed seeds in the soil waiting for the right conditions to arise for them to germinate. Those conditions may be related to warmth, water or light levels. So, our very cultivation of the soil, bringing seeds to the surface towards sunlight and water, can trigger germination.
Another way that annual weeds gain an advantage over other plants is by producing huge numbers of seeds. Sowthistle for example, can produce between 20,000 and 25,000 seeds in its single year’s life-cycle. Chickweed, on the other hand, will produce only around 2,000 to 3,000 seeds per plant, but then Chickweed’s life-cycle is only 7 weeks long, so the total number of seeds that it can produce in a year is enormous.
So, you will be getting the picture that with annual weeds, the battle revolves around the seeds.
Many perennial weeds also flower and set seed, taking advantage of the sorts of tactics outlined above in relation to annual weeds. But perennial weeds have the additional advantage of living longer. This has enabled them to develop some other effective ways of ensuring that they thrive and survive.
Perennials can reproduce vegetatively as well as sexually. This enables them to spread out and colonise the ground around them and to survive if parts of the original plant are removed or damaged. This process of vegetative reproduction is associated with various characteristics of perennial plants roots and stems, as detailed below. Most weeds only have one of these characteristics but some have more than one and those, unsurprisingly, are among the most persistent weeds that we find.
It is worth noting at this point that these methods of reproduction are not exclusive to weeds. They are common to most species of perennial plants and as gardeners we very often taken advantage of them to increase our stocks of desirable plants. The concern for us, is that given the right conditions, weeds are able to use these methods of reproduction, quickly and aggressively, thus taking over large areas of our gardens and overwhelming our plants of choice.
Weeds like Dandelion and Dock have long tap roots that reach deep below the surface of the soil. These roots are almost impossible to remove and enable the plant to regenerate if the top section of the plant or even part of the taproot is removed. They are also highly effective storage organs and can help the plant regrow if unfavourable surface conditions kill off the top growth .
Other weeds have roots that spread out horizontally, acting like rhizomes (see below). Whilst others, such as bindweed, have roots which grow out huge distances underground (up to 25 meters away) and which can send up new plants if cut or damaged.
Stolons or runners are shoots or stems of the plant that contact the soil and give rise to the growth of a new plant at the point of contact. Some may be stems that arch over under their own weight, as is often the case with blackberries, while some may grow out horizontally. Where the nodes of the stem touch the soil surface, roots and shoots can grow. As the new plant develops at that point it may itself send out runners, thus perpetuating the outward spread of the plant. Even some annuals adopt this tactic, but it is often low growing perennial weeds, such as Creeping Buttercup, that use it as their major means of colonising their surroundings.
Rhizomes are the underground equivalents of runners. Stems, rather than roots, they are present in weeds such as Couch or, in warmer climates, Bermuda Grass. Rhizomes can spread rapidly in cultivated soil, producing roots and shoots at each node and enabling the plant to easily taking over its surroundings in no time. The big problem with both runners or rhizomes is that each section of the stem that contain a node has the potential to produce a viable independent plant. This means that cultivation or tilling of the soil that cuts these stems into smaller pieces merely aids the plant’s reproductive efforts.
Bulbs are efficient storage organs that reproduce by developing a succession of new bulbils alongside the parent bulb. We of course welcome this process in daffodils and tulips as the clumps become larger and we are able to dig them up to split off the new bulbs to plant elsewhere. In weeds, this process enables the plant to reproduce endlessly, especially where the bulbils split off easily from the parent when the plant is disturbed.
Corms are similar to bulbs, although each year’s growth produces a new corm on top of the old one. Nevertheless, the old corms remain viable for some time, so if the succession of corms are broken up, each may produce a new plant.
In addition to the reproductive techniques, many weeds have also evolved defence mechanisms that help them survive attack. These adaptions will have evolved to protect the plant against attack from animals but many are equally effective against humans. So, for example, some weeds such as Dogbane (Apocynum cannabinum), Deadly Nightshade (Atropa belladonna) and Milkweed (Asclepias spp) can be highly poisonous. Some, such as Blackberry (Rubus spp) and Aparagus Fern (Asparagus aethiopicus) defend themselves with sharp thorns. Others, such as nettles (Urtica dioica), deliver stings, whilst plants such as Poison Ivy (Toxicodendron radicans), Poison Sumac (Toxicodendron vernix) and Giant Hogweed (Heracleum mantegazzianum) can cause severe skin rashes on contact with their sap.
As you can see, all of these characteristics enable these weeds to hold their ground against animals or humans who might remove them and therefore give further explanation for the success of these plants in colonising ground.
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