Persistent weeds causing you grief ?
Perennial weeds can be challenging to control
“... BUT in this world nothing can be said to be certain, except death and taxes.”
American Benjamin Franklin’s famous words are a fair indication that, whilst this Founding Father was a noted scientist, printer, writer, statesman, and inventor, he was certainly NOT a gardener!
Why? Any world-famous gardener worth their salt would have said, “…except death, taxes and WEEDS!”
Weeds are one of the certainties of any horticultural pursuit, totally non-discriminatory life-forms that invade broad-acre farm paddocks, commercial orchards, hobby farms, backyard vegie patches, ornamental beds, annual displays, lawns and even that single potted plant on the front porch.
Many weeds are fleeting annual aberrations in our landscape, that is, they germinate from seed, grow, flower, fruit and seed again in a short seasonal life-cycle.
This can be somewhat of a mixed blessing, as the clever gardener can attempt control by removal or spraying early in the weed’s life to reduce the next seed bounty.
This works well for common annual weeds such dead nettle, chickweed, pigweed, bindii, caustic weed and milk thistle.
Some can be downright annoying, for example wintergrass, due to its ability to grow, flower and seed in as little as three to four weeks, leading to many life-cycles and copious seed in a season.
But by far the most tenacious weeds are those that use multiple methods to recreate and survive, doing so over many years, the so-called “perennial” weeds.
Arguably the worst of all perennial weeds is the much-loathed nutgrass or nutsedge (Cyperus rotundus).
This abhorrent plant has been included in the “world’s worst weed” list, spreading to around 90 countries since escaping from Africa, southern Asia and Europe.
Its common name relates to the many round tubers, or “nuts”, produced on a tangle of underground rhizomes, some as deep as 40cm, in all types of soils.
Every tuber (even part of a tuber!) can produce many hundreds of plants, nearly 100 new tubers, and spread up to 3m, in one season.
To make matters worse, the rhizomes and tubers release chemicals which can inhibit growth in other nearby plants.
Farmers hate this plant as their cultivation practices simply spread the tubers further each season.
Gardeners despise this plant as it pops up in our vegie patches, flower beds, lawns, pathways and even through weedmat and plastic mulch!
And how many of us has had a load of “clean” soil or mulch delivered only to have this despicable weed emerge soon after the dust has settled? So how do we rid our gardens of this pest plant?
Forget physical methods, unless it’s removal of a deep layer of “nutty” soil with a bobcat or excavator.
Some gardeners believe that persistent removal of plant tops will gradually weaken the plants, but this is slow and tedious.
The best control method is via chemical application. Spraying with glyphosate around flowering time (autumn/winter) works well, but application of products containing halosulfuron-methyl, which targets plants in the sedge family, will produce excellent results. Spray only actively-growing plants for best results.
Another perennial weed infesting nearly every part of our gardens is creeping oxalis (Oxalis corniculata).
This little blighter, called creeping woodsorrel in some parts of Australia, can form dense colonies in lawns, suddenly appear through the top of groundcovers, low shrubs and throughout vegie patches, and quickly spread from pot to pot in gardens or nurseries. What makes this such a nasty weed to control?
Simply put, it attacks on three fronts. Firstly, every plant produces many small, yellow flowers followed by seed pods which, when dry, explode when disturbed by anything or anyone. Seeds can be catapulted up to two metres from the parent plant.
Secondly, every plant spreads laterally via stolons which root at every node that touches soil. A single plant can spread up to one metre in a season.
Thirdly, a deep tap root is produced by individual plants, and, even if the top growth is removed, the tap root will regrow. A very persistent weed indeed!
In gardens and pots, physical removal of small plants is one method of control, but it must occur before a tap root grows and before flowering.
Once this weed spreads through other plants via stolons, physical control becomes difficult and any chemicals applied may damage surrounding non-target foliage.
This is one tough cookie when established amongst garden plants and pots! However, control in lawns is easier via products containing MCPA and bromoxynil. Make sure to use a wetting agent to help the product stick to leaves.
Perennial “nasty” number three, onion weed (Nothoscordum inodorum / Allium triquetrum) is usually confined to cultivated areas in our gardens. Looking very much like a garlic plant, this weed uses two methods to invade and conquer. Firstly, much seed is produced after flowering.
This can be spread by wind or unsuspecting gardeners. Best control method is to snap off the flowering stems before they mature. Secondly, and much harder to control, is this plant’s ability to produce many small bulbils on the main underground bulb.
Some plants can have up to 50 loosely attached bulbils, which, when the plant is disturbed (via cultivation or attempts to remove), will detach and grow into new plants next season.
These new plants then have the ability to produce bulbils, and so on…..well, you get the picture!
Physical removal will only work if a generous amount of soil containing the bulbils surrounding the plant is also removed.
Best results to control this weed will be gained by spraying young, actively growing plants with glyphosate.
Repeat applications will be necessary to control bulbil regrowth, as they may not be affected by application to the parent plant.
If you have any great perennial weed control strategies, email me at wellsleyhorticultural@gmail.com and I’ll be happy to let other gardeners know.