I guess the start of a conversation around weed management should cover what a weed actually is, some form of definition. Many years ago an old mate gave me his definition of a weed and he had an interesting take on things:
“You pull a plant and toss it onto your compost heap, if it dies it was just a plant, but if it survives it is a weed!” – Tim
Personally this seems a bit too much like the witch trials for me but Tim did have some interesting ideas! Looking a bit further afield, here are some definitions provided to me by my good friend Uncle Google –
- Oxford Languages Dictionary - "a wild plant growing where it is not wanted and in competition with cultivated plants".
- Merriam-Webster - "a plant that is not valued where it is growing and is usually of vigorous growth"
- Cambridge Dictionary - "any wild plant that grows in an unwanted place, especially in a garden or field where it prevents the cultivated plants from growing freely"
- Collins Dictionary - "a wild plant that grows in gardens or fields of crops and prevents the plants that you want from growing properly".
- Or my favourite from Wikipedia - "a plant considered undesirable in a particular situation, growing where it is not wanted".
Putting that in simple parlance: a weed is a plant out of place. It does not have to be a wild plant to cause problems, as anyone who has done battle with the rootstock of a Nelly Kelly passionfruit can attest!
So, having seen that the general characteristics of a weed are that it is just a plant growing in an area where you don’t want it to be, why is that a problem? There may be any one of a number of problems associated with that plant out of place -
1. Competition – weeds will compete with the plants which you are trying to grow to a greater or lesser extent. They will compete for light, water, nutrients and/or space all depending on what your main crop is and how invasive the weed is. With the wet weather we had around here over the past couple of years, I found that commelina cyanea (scurvy weed) moved in and took over a number of our patches. It was fast growing and was physically able to outcompete a number of veggies for light. At the least a bad weed infestation can result in reduced yields, with death of crop plants as a worst case scenario.
Weeds can develop into a mass of foliage, competing with crop plants for nutrients, water, space and light
2. Hosts for Pests – Weeds can act as hosts for pests of crop plants providing a place where they can feed and reproduce, camouflaged in amongst all the weedy growth. Also, pests can overwinter on weeds so they are ready to start up the following spring. Some examples include thrips on a number of weedy species, aphids on sow thistle (sonchus species) and harlequin bugs on long grass and herbaceous weeds.
3. Host for Diseases - Weeds can also act as host for crop plant diseases such as viruses that can then be transmitted to crop plants by sucking insects like aphids, whitefly, leafhoppers and thrips. Some diseases such as the tomato spotted wilt virus and Cucumber mosaic virus are hosted by a wide number of weed species. Shepherds Purse (capsella Bursa-pastoris) can also host a wide number of insect, nematode, viral and bacterial pests that may damage crop species.
Jasmine - an invasive weed with associated allergy problems
4. Allergies – the pollen produced by many plants, including weeds, can cause human allergies. My daughter is very allergic to jasmine (jasminium species) when it flowers. Other common weeds that can cause allergies include – amaranth (amaranthus species), dandelion (taraxacum officinale), dock (rumex species), fathen (chenopodium album), nettle (urtica species), plantain (plantago species), red sorrel (rumex acetosella) and rye grass (lolium rigidum).
5. Physical injury – Caused the pointy bits (technical term) of weeds such as stinging nettles also giant, scrub and dwarf nettle (urtica species), thistles, like spear thistle (Cirsium vulgare) or scotch thistle (Onopordum acanthium), or bindii (Tribulus terrestris). There is nothing quite like leaning down to grab some tomatoes and getting a handful of stinging nettle, it certainly presents an opportunity for the neighbours kids to learn some new words!
6. Dermatitis - Some weeds can also cause dermatitis on skin contact, either immediately or after repeated contact such as parthenium weed (parthenium hysterophorus), petty spurge (euphorbia peplus) or cotton bush (Gomphocarpus fruticosus).
Petty Spurge (euphorbia peplus), a common weed around here, has a number of hazards associated with it
7. Poisoning – Some weeds are poisonous and can cause problems if harvested as with vegetables, eaten by children, or eaten by pets such as chooks or rabbits. Some poisonous weeds include include apple of Sodom (Calotropis procera), cotton bush (Gomphocarpus fruticosus), blackberry nightshade (green berries) (solanum nigrum), Deadly nightshade (Atropa belladonna), green cestrum (Cestrum parqui), pellitory (Parietaria Judaica), petty spurge (euphorbia peplus) and thornapple (Datura stramonium).
8. Allelopathy – This is a property of some weeds where they produce on or more chemicals that can inhibit the germination of seeds and/or growth of the crop plants surrounding them. Some examples of weeds that can have this effect are - variegated thistle (Silybum marianum ), purslane (Portulaca oleracea) , fathen (chenopodium album), white clover (trifolium repens) and red clover (trifolium pratense) and garlic mustard weed (Alliaria petiolate).
9. Cross pollination – while this will not be a problem unless you want to save your own seed, in some cases wild plants (weeds) can cross pollinate with vegetables of the same species, resulting in lower quality vegetables when those seeds are planted. Examples are Queen Anne’s lace (daucus carota) and carrots; wild turnips (brassica rapa) with other brassica rapa species such as pak choi, mizuna and mibuna
10. Aesthetics – depending on your point of view, a weedy place may look natural and attractive, or they may look messy and untidy, particularly if they are growing in amongst your prized rock garden or flower bed. In that case you may want to look at some weed control strategies.
Natural and attractive or messy and untidy? Your call!
So there are a whole stack of reasons why you might want to control weeds on your property. Clearly before you can do that, you need understand what weeds you have to deal with. So the next part of the process is how to identify and record which weeds are weeds to you!
Weed Management Par 1: Knowing Your Weeds
