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by Diana Roberts
Category: Disease-Pests
A few weeks ago, I was called to take a look at the spread of some sort of weed on top of a gravel driveway. The homeowner explained to me that they had noticed a small area of strange looking weed-type growth last year, and then all of a sudden this year it was covering a huge area of their driveway!
The problem area was on the north side of the owner’s home. It looked to me like a weed which had frozen and turned black and slimy, but the homeowner assured me that this is the way it looks whenever it has been raining. When the weather is hot and dry, the problem takes on a different look; it becomes dry and crusty, cracking and can be peeled off. Never having seen this, I pulled out my problem-solving book and got to work. It looked like algae to me, you know the stuff that washes up on the shore of lakes and oceans.
What I discovered was that it was indeed algae! This type of algae is a freshwater plant that will invade any shady, wet area of a lawn or bare ground. If you discover algae on your lawn, you will find that it is smothering the crowns of the plants. If you walk over it, it is very slippery. The algae will be carried across the area on the bottom of your shoes or on the feet of animals as well as on the tires of equipment and vehicles. Driving on top of the algae all summer had spread it like wildfire.
Lake water
What the algae needs to live and spread is compacted soil (which is why a driveway is perfect), high nitrogen content in the soil and lots of organic matter as well as very wet conditions. Organic fertilizers will also encourage the growth of the algae in the spring and fall. The area where our homeowner’s algae started is where a boat was parked the year before, so it is possible that the algae originated from a lake. If you water your lawn from a lake, stream or pond, this water will usually contain algae.
To start to rid your lawn or driveway of algae, you will need to spray the area with wettable sulfur, then a month later spray the area again. The conditions which are favorable for the algae must be changed or it will return.
For lawns, you should reduce the soil compaction by aerating. This is something you should do consistently to keep a healthy lawn. Aerating can be done simply with a garden fork poked into the ground over your entire lawn, or by wearing aerating shoes while mowing your lawn. For a large lawn, you may want to use an aerator attachment on the back of a riding mower. Professionals use plug aerators which leave narrow holes in the lawn and plugs of soil on the top (which look like goose droppings). Do not top-dress your lawn with compost or high organic matter as this will only encourage the problem. You should also stay away from organic fertilizer for a few years.
Any area which has a problem with algae should have the drainage corrected if there is a problem. Get rid of any tall trees which may be keeping the area shaded and damp. As I said, the property where I saw the algae was on the north side of the home which will keep it shaded no matter what. It will continue to be compacted because it is a driveway and that cannot be changed. The one thing I noticed was that the road was above and sloped down to the driveway, which means that any rain or spring runoff will flow onto the driveway. This is a problem which can be overcome by digging a ditch where the road meets the driveway allowing the water to flow away from the area.
The most important thing to do is keep an eye open for any ‘new’ weeds and deal with them immediately. A small patch of algae is much easier to get rid of before it has spread across the whole area. Don’t think that it’s not a problem if it is only on your driveway, because algae will spread to every area of your landscape if the conditions are right.

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