Plant Doctor Archive

Algae on gravel

I HAVE problems with a fungus that is growing in my gravel drive. I've sprayed it with Clean Up but that didn't work, so I tried Roundup which killed some of it, but after the rain it's now growing back. What is it and how can I get rid of it?

THE sample you sent was not a fungus but one of the many types of algae that crop up at this time of year. These algae are very simple plant-like organisms, closely related to pond slime and seaweeds, which thrive in constantly moist, shady situations. In the garden they look unsightly, but can be positively dangerous on hard surfaces like concrete paths or steps as they become extremely slippery.

Once the weather dries up the problem will probably resolve itself, but the spores will always be around and algal growth will return in moist conditions. Luckily, they are relatively easy to kill off on a gravel path, being susceptible to most household disinfectants, including bleach. As long as conditions are favourable the spores will usually die off within a few hours of being sprayed. Be careful not to get these solutions on nearby plants as some will cause damage.

There are several other products available you could choose from, like No Moss Mould Mildew, Yates Moss Killer, Surrender and Wet and Forget, which among their other attributes also control most algae. As a rule, heavy rain a day or so after spraying will reduce the effectiveness so you may have to apply it again.

After the algae has died off you'll need to do something to deter it from returning and with a gravel path, one way is to simply rake the gravel regularly. This turns the stones, exposes them to sunlight, encourages drying and also breaks up any algal growth before it gets too advanced. Increasing the depth of gravel could help, too, as there is less chance of algal growth on gravel that is not in direct contact with damp soil.

Reproduced with permission from the former Weekend Gardener magazine. The views expressed here are not necessarily those of the RNZIH.