Weed control through chemicals
The key to success with herbicides or weed killers is the timing of their application. When plants are young, at the seedling stage, when their growth is active and vigorous then water, sugars, amino acids and other materials are being moved around the plant in the transport systems (the xylem and phloem). If a herbicide is applied at this stage then it, too, is likely to enter into the plant’s systems and have a overwhelming effect.
Established perennial weeds may need to be treated several times with a systemic (see below) weed killer; again it is best to do this when the plant is in an active phase of growth – when the sugars made in the leaves are passing down to the roots and / or storage organs. The sugars and amino acids move in the phloem (sometimes termed bast), whereas water flows through the xylem from the roots up to the leaves.
There are various types of herbicides. Some work on both broad leaved (net veined) and narrow leaved (parallel veins – like grasses) plants; whereas some only work on the broad leaved plants. Contact herbicides work by killing off the top growth of a plant, but may leave behind a working root system – which can regenerate leaves and stems; this is especially true of perennials. Again, such herbicides tend to be most effective when the plant is in an active phase of growth. Examples of contact herbicides are : Ammonium octanoate / decanoate, ammonium sulphamate, paraquat / diquat and sodium chlorate. These act in various ways from destroying the photosynthetic mechanism to damaging cell walls.
Systemic herbicides work in a somewhat different manner, they are taken up by the leaves and transported through the phloem to roots and underground storage organs / tissues. If the plant is again in an active phase of growth then this tends to ensure that the foliage is extensive which helps the uptake of the the herbicide.
An example of a systemic herbicide is Glyphosate; this stops the plant from making amino acids, which are needed (in turn) for making proteins. Proteins may function as enzymes - biological catalysts, which control all the reactions that go on in the plant. Glyphosate is not necessarily 'a quick fix', it may take some days or a week or so for its effects to become apparent (or longer in cold weather).
Amitrole or amino-triazole works in a different way - through the breakdown of chlorophyll, the photosynthetic pigment. With the loss of chlorophyll, the plant cannot make sugars, starch and other organic compounds needed for growth and development.
