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Why did that weed come back?

Published

July 3, 2024

Author

The Searle Gardening Team

A question many gardeners wonder after they have sprayed a selective herbicide, see it wither and die and a month or two later spy it sprouting again. It is not the original plant you sprayed, but its offspring.

With selective herbicides (the ones you use on lawns), it will state on the packet which weeds it will target and kill. These weeds take in the herbicide and die, and it can take up to two mowing cycles. That means if you mow weekly it will take two weeks to have an effect and if you mow every three weeks you will see its affect in six weeks. You spray, they die.

Now a few months later you see them again, these are the seeds they produced, now sprouting. Every time you see a clover, creeping clover, bindi, dandelion, oxalis, or thistle growing, it is often the flower that draws your attention. Those flowers quickly turn to seed and spread through the lawn. They hitchhike on shoes, mower tyres, blown in or from birds, insects or animals ‘bringing in seeds. Those seeds can sit dormant until the conditions suit them with warmth or rain to start to germinate.

It is not the parent plant that has resprouted but the thousands of seeds they have distributed. That is why on many selective herbicides they may say to respray at a set time.  This is to get the germinated weeds before they are mature enough to develop seeds themselves.

Have you ever noticed the creeping oxalis, the yellow flower with a clover like leaf in your pots, gardens, and lawns. They have explosive seed pods, which contain 5 compartments and within each compartment there are several seeds, this is just from one flower alone. It only takes one flower to ripen and they fling the seeds with force to start the chain of events. This is why they are so hard to get rid of, you kill the parent plant but the seed can sit and wait for optimal conditions to torture you again.

This is why it is important to spray weeds before flowering and seeding to break the cycle. For the majority, it is a repeated process but you will see the numbers decreasing over time.