On my back barn hill, tons of burdock and motherwort went to seed before I noticed. I removed the stocks to burn but heard thousands of seeds drops in the removal…
Will occulation kill off the seed bank or will occulation kill the rossettes if they grow in the spring? I am planning on rolling a heavy rubber mat and leaving it until spring to hopefully kill seeds..
After burdock goes to seed will it die? If I am able to cut back yhe stocks every year will I get rid of the burs?
Thank you for contacting the Toronto Master Gardeners about your problem with burdock (Arctium minus) and motherwort (Leonurus cardiaca). What a challenging situation you have! A single burdock plant can produce 15,000 seeds, and these seeds can remain viable in soil for up to 10 years. Burdock spreads only by seeds, however motherwort spreads by seeds and underground rhizomes. Burdock is considered to be a biennial. It forms a rosette of leaves in its first year and flowers then sets seeds in its second year before dying off. However it often behaves more like a perennial in that it takes several years to produce flowers (competition from other plants can cause this) before dying off. Burdock has a very long and tough taproot. It is generally recommended to cut back the rosette in it’s first year and then hand pull or dig out the root entirely (maybe not feasible in your case).Motherwort is a perennial. It can spread very quickly via underground rhizomes and form dense clumps. To stop it from growing and spreading every bit of the roots must be removed.
Occultation (the use of opaque tarps) blocks the sunlight that plants need to grow. It is effective for killing vegetative growth (like the burdock rosettes) usually in 4-6 weeks. There is very little information about the effectiveness of occultation on killing the seedbank. It seems that the most likely outcome is fatal germination, meaning that occultation increases soil temperature which causes seed germination and then seedlings are killed due to lack of sunlight, thus depleting the seedbank. It is unlikely that all seeds would germinate at once.
While occultation kills vegetative growth fairly quickly, it will take much longer for the roots of perennial weeds to die. The underground storage structures of perennial weeds store enough energy reserves to survive winters and other difficult conditions which makes these plants resilient to tarping. There is very little information about how long to leave tarps in place to kill perennial weeds, however it has been suggested that 18 months would probably be effective. It has also been suggested that the best time to put down tarps to kill perennial weeds is when they are most vulnerable, that is when they are just emerging in the early spring, because they have already drawn on reserves to survive the winter and then must further exhaust their reserves to form spring shoots.
Regarding cutting back burdock flower stalks, t is crucial that these are cut down before seed set. Burdock typically flowers from July – September in Ontario. The same would apply to motherwort which flowers from late May through August. Mowing can also be effective for preventing seed dispersal and suppressing weeds. Perennials with well-established root systems will not be killed by mowing but you might be able to starve them by repeatedly cutting off new growth at ground level.
Hopefully this information will be helpful as you tackle your very challenging weed problem. Best of luck!