Generally, the term “weed” is used to describe any plant that is unwanted and grows or spreads aggressively. The term “exotic weed” describes an invasive unwanted non-native plant. Terms such as invasive weed or noxious weed are used somewhat interchangeably to refer to weeds that infest large areas or cause economic and ecological damage to an area. The term “noxious” weed has legal ramifications in some states that maintain official lists of noxious weeds.
So, what is considered a weed in one area may not be a weed in another! For example, the owner of Lawn Green has a stunningly beautiful “kikuyu” lawn at his home, however, for someone else this “kikuyu” could be a living nightmare that is overtaking his garden.
Put simply, a weed is an unwanted plant.
Weeds can be grouped into several broad categories.
- Grasses (for example, kikuyu, couch, buffalo, St. Augustine)
- Grassy weeds (for example, paspalum, wintergrass, summergrass, crabgrass)
- Sedges (for example, nutgrass, mullimbimby couch)
- Broadleaf weeds (for example, bindii, clover, dandelions, oxalis, buttercup, thistle).
There are many practices the home gardener can adopt to keep on top of the continuous battle with weeds. These practises can be split into three groups:
- Physical weed control – physically removing weeds from around the home garden and lawn may be the only way to remove some weeds species (for example, removing couch or bermudagrass from mondo grass; or wandering dew from garden beds or lawns), however, this may be a very time consuming. It may be the only way to remove some weed species that existing or current herbicides have little or no effect on. Also, the weeds may not be safely accessible with a herbicide spray because they are growing in amongst other desirable plant species. When physically removing weeds it is important to remove all plant parts including the roots, any stolons (above ground runners), rhizomes (underground runners), bulbs and vegetative pieces, as some weed species are capable of regrowing from any of these plant parts if they are left behind. Removing weeds before they flower and set seed will help reduce the build up of future weed populations.
- Cultural weed control – adopting various cultural practices around the home garden is probably the best long term weapon the home gardener has in the fight against weed invasion. A healthy and vigorously growing lawn is a big suppressant of weed growth, because it acts as a barrier to and shades out emerging weed seedlings.
The following cultural practices include:
1. regular lawn mowing (weekly during the warmer months and every seven to fourteen days during the cooler months depending on the day/night temperatures in your region).
2. frequent mowing encourages the grass to spread out and thicken (good lateral growth), thereby, suppressing weed emergence and reducing the chance of tall growing weeds to set seed.
3. mowing the lawn at the correct height for the species grown. If the lawn is cut too high it will not spread and thicken and if it is cut too short it will scalp the lawn resulting in a less vigorous growing habit and therefore reduce its ability to compete with weeds.
4. regular fertilising to encourage a healthy, vigorous lawn.
5. garden mulching (to a depth of 50-100 millimeters) and planting out with ground cover plants are very effective ways of suppressing weed growth
- Chemical weed control – Herbicides are the common alternative to hand weeding for controlling weeds in the home garden. A herbicide is a chemical that kills weeds or suppresses weed growth.
The chemicals in herbicides are referred to as active constituents. The active constituent is named on a herbicide package or container directly under the trade name and is measured in grams/Litre (g/L). It is not uncommon for a herbicide to contain more than one active constituent and by referring to the active constituents listed on the package rather than the trade name you can compare products by what they contain and their concentrations.
Herbicides can be split into two groups, selective and non-selective. Selective herbicides control the weeds listed on their label but will not damage the crop that they are registered for use in e.g. the active constituents MCPA, dicamba and halosulfuron methyl are registered for use in turf. Non-selective herbicides control the weeds listed on their label but can damage or kill other plant life that they come into contact with e.g. the active constituent glyphosate.
The information in Table 1. sets out the range of active constituents registered for use in the home garden and some of the many trade names under which these active constituents can be found.
Herbicides have different modes of action depending on whether the herbicide is a contact herbicide, a systemic herbicide or a residual herbicide.
A contact herbicide (e.g. active constituents bromoxynil, sodium chloride and pine oil) only kills the plant part that it comes into contact with, so good coverage of the whole surface area of the weed is essential for good weed control. The disadvantage of herbicides with this mode of action is that the under ground part of some weeds, particularly those that have under ground storage bulbs e.g. onion weed, can remain unaffected by the spray and regrow soon after spraying.
A systemic herbicide (e.g. active constituents MCPA ,dicamba and glyphosate) once sprayed onto the target weed is translocated through the plants vascular system, killing the above ground foliage and the roots. For best results the target weeds should not be undergoing any type of stress e.g. from waterlogged soil, dry soil, frost damage or extreme heat, as this will hinder the translocation of the herbicide. Good spray coverage will improve the efficacy of systemic herbicides.
A residual herbicide (e.g. active constituents imazine) is applied to bare soil and then incorporated into the soil soon after application, either by rainfall or with the irrigation sprinkler. Residual herbicides reside in the soil for several months and controls any plant that germinates in the treated soil band, either before it reaches the soil surface or very soon after. The length of time the herbicide remains active in the soil depends on the rate of application, the susceptibility of the emerging weed to the herbicide, the rate of breakdown from heat and microbial activity and how quickly the herbicide is leached through the soil. Residual herbicides are very useful for controling weeds growing in paths, driveways and pavers. However, care must be taken, as any desirable plant thats roots come into contact with the herbicide may be damaged or killed.
Weeds in garden beds may be spot sprayed with a herbicide if great care is taken not to spray any desirable plants. If the herbicide does make contact with a desirable plant it should be washed off immediately with water to reduce possible damage. The active constituents glyphosate and pine oil can be used for spot spraying in garden beds as they don’t leave a residue in the soil.
There are a few precautions that should be taken when applying herbicides in garden beds.
- It may be necessary to adjust your spray nozzle to produce large droplets rather than a fine mist spray, as large droplets are heavier and are therefore less likely to drift onto desireable plants.
- Keeping the spray nozzle as close to the target weed as possible will also reduce the chance of drift. A small paintbrush may be a safer alternative for herbicide application than a sprayer when applying to weeds in very close proximity to desirable plants. Care must be taken not to flick the paintbrush.
- Never apply herbicides with a sprayer when wind is strong enough to cause a drift hazard.
Herbicides are a very efficient way of controlling weeds in the home garden lawn. For maximum affect on the weeds and greatest safety to the lawn, the active constituent in the herbicide needs to be active on the weed species present and safe to use on the lawn species in question. All of this information is displayed on the herbicide label.
The majority of broadleaf weeds in the lawn can be controlled with a range of selective herbicides. However there are fewer selective herbicide options available for the control of grass weeds and sedges in the lawn. Summer grass, barnyard grass and paspalum can be controlled with selective herbicides containing the active constituent DSMA, while nut grass and mullimbimby couch can be controlled with selective herbicides containing the active constituent DSMA, or halosulfuron methyl.
It should be noted that selective lawn herbicides may damage soft new lawn growth that occurs after fertilising or top dressing. Herbicides can be safely applied after this new growth matures. This takes about four weeks.
Paths, pavers and driveways
Weeds growing through cracks in paths, pavers and driveways can become a real nuisance. To overcome the repetitive task of hand weeding or spraying with a contact or systemic herbicide a residual herbicide can be applied. Residual herbicides when applied to bare dirt, pavers and cracks in paths and driveways can remain active in the soil for up to 12 months, making hand weeding of these sprayed areas a non-event instead of a monthly one.
When using residual herbicides, there are a few important rules that need to be followed, to ensure maximum weed control and minimum damage to the surrounding desirable plants.
1. Before application, any existing weeds must be removed so that a bare surface is left to spray. After application the herbicide needs to be watered in, either by rain or with the sprinkler, to move the chemical into the soil band where weed seeds germinate. Weed seeds that germinate below the chemical band will not be controlled.
2. Residual herbicides should not be applied to areas where desirable plants/lawn are growing, or will be planted within the next 12 months.
3. While spraying the herbicide do not allow it to drift onto desirable plants.
4. Residual herbicides should not be applied to areas that directly drain onto lawn or garden beds.
It is important to use all of these practices together and not rely on just one to achieve optimum results in weed control.