Every year, new and different herbicides arrive on the market. And every year farmers buy the herbicides and faithfully apply them to the targeted weeds. And if not next year, then eventually, herbicides will have to be applied again to the same weed species. Why are the weeds so successful in doing battle with farmers? Because the most difficult weeds to keep under control have developed an army of resources enabling them to attack on all fronts.
Let's take a look at some of these weeds. The weed that raises every farmer's dander, direct seeder or not, is Canada Thistle. It has a whole arsenal at its bidding to do battle with farmers. It produces thousands of seeds in a single year. One plant can produce up to 5000 seeds and those seeds will remain viable in the soil for 20 years! Its horizontal roots can spread 6 metres as can its vertical roots. Altogether, the roots can produce another 500 metres of new root!
Another weed capable of creating anxiety is scentless chamomile. Pretty little flower to look at along the roadside but a migraine maker if it's in your field. One scentless chamomile plant is capable of producing half a million seeds (I hope some little summer student wasn't asked to sit down and count the seeds one-by-one!). It also has a dense and fibrous root system. And as anyone who has it on the farm knows, scentless chamomile germinates throughout the year because it has a winter annual habit, summer annual habit or short-lived perennial habit.
Stork's bill is another one of those weeds that has numerous growth habits. It likes to emerge in early spring right about the time the crop is also emerging. And its tiny little seeds are difficult to separate from the crop seeds at harvest time.
Field horsetail comes in a variety of forms. The interesting thing about this little weed is that, taxonomically, horsetail is considered a primitive plant! Since it's survived this long, it's obviously very well adapted. Not much research has been done on horsetail in terms of how to control it. Tillage doesn't seem to hurt it much and the plants don't seem very bothered by herbicides, either. However, there are reports of yellowing and a set back after a dose of Roundup. The plant's unusual leaf structure and silica content may be the reasons for its ability to overcome control measures.
Field Bindweed is a perennial. It reproduces by seed and root buds. It has a primary tap root from which lateral roots will develop. These lateral roots can persist even if they are severed form the tap root. Due to the rough texture of the stem and the leaves, the plant is difficult to wet, as with herbicidal sprays.
Foxtail Barley is a perennial with fibrous roots. It reproduces by seed but also has the ability to regenerate itself from little buds in the crown just below the surface of the ground.
Black Medic is another one of those weeds with a variety of habits including annual, biennial and short-lived perennial. Its slender tap root usually has several branches. As the plant continuously produces flowers, it ensures that seeds are available at all stages of maturity at all times during the growing season.
Downy Brome is mostly considered a winter annual although it can also be a spring annual or biennial. Its finely divided fibrous root uses moisture from the upper layers of the soil thereby making it difficult for other species to establish. Its root has the ability to grow throughout the winter so when temperatures warm, the plant has a head start on other species. Downy Brome reproduces by seed and is a prolific seed producer. It matures early in the growing season.
Weeds are a formidable force with which to reckon. Whatever method man employs to eradicate the weeds, the weeds are able to adapt and maintain their position. The war between weeds and man is unlikely to ever end and so far, the weeds have the advantage.