Dethatching and Aerating Lawns       Bookmark and Share

Excessive thatch-more than ½ inch has become a problem for many high maintenance lawns in Minnesota. Thatch is a tightly interwoven layer of living and dead tissue between the green vegetation and the soil surface. It is composed of products from stems, leaves and roots that are resistant to decay. A little thatch improves the wear tolerance of a lawn, too much can harbor disease organisms and insects making the lawn more susceptible to damage from disease and drought. Thatch buildup and soil compaction are both conditions that can cause lawns to struggle. When they are severe, they can ruin a lawn. Luckily, both are problems that can be resolved. Simply dethatching or aerating the lawn will provide an immediate fix, but it is also important to find out what the contributing factors are and make some changes so that the solution will be long-term, not just temporary.

To see thatch clearly, simply dig out a scoop of your lawn (don’t worry, you can put it right back when you’re done) and look at it from the side. You should be able to clearly identify the soil, the thatch and the top growth of the lawn grasses.The thatch layer will look like a tightly knit layer of brown debris. In a healthy lawn, the thatch layer will decay naturally,
providing nutrients for the lawn. Having no thatch layer isn’t the goal either, because that can make the lawn more susceptible to drought and heat stress. Thatch not only provides nutrients, it also helps shade and protect the crowns of the grass plants and helps conserve moisture. Lawns with a healthy layer of thatch do not need dethatching. Sometimes the lawn can have too much thatch. Excessive thatch most often happens when the lawn is fertilized too often, the grass clippings are too long or there aren’t enough of the microorganisms that digest the thatch present. Most often, it
is a combination of these factors. When the thatch layer is too deep (in excess of a half inch), the lawn will begin to thin. Lawns with heavy thatch tend to have a shallower root system, making the grass harder to maintain. Thatch can keep water, air and nutrients from getting to the roots. And if that isn’t enough, lawns with excessive thatch are also more likely to have problems with diseases.

All About Dethatching
Dethatching is the process used to remove the excess thatch. It is best to dethatch the varieties of grass grown in our area in early fall. When lawns are dethatched in spring, there is a much greater risk of damaging the new grass crowns. If a severe thatch problem is discovered in spring, you are faced with deciding if more damage will be done by dethatching in spring or by leaving the thatch on the lawn for the summer. When it is time to dethatch, mowing the lawn fairly low just before starting will make the job easier. Hand raking is a backbreaking way of removing thatch from any but the smallest areas. There are several better ways to accomplish this task. There are mechanical dethatchers. These machines are usually rented for the day or many lawn care companies offer this service. The best type of machine for dethatching is a vertical mower. It has a row of blades that cut groves into the thatch and pull it up. The other alternative is a power rake. They are like lawn mowers with spring tines attached to the blades. Power rakes work, but they don’t pull up as much thatch and they pull up more grass than a vertical mower, sometimes damaging the lawn. If you are using a mechanical dethatcher, it is better to set it too high than it is to set it too low. Since you almost always have to go back over spots several time the blades can be lowered once you become accustomed to working with the machine. Once the thatch is pulled up to the surface, it will have to be raked up and either composted or disposed of. Do not allow it to sit on the surface of the grass for very long. There are also liquid products that can be applied to the lawn to speed up the breakdown of thatch. They are not as reliable as mechanically dethatching and should only be considered to aid digestion on a healthy lawn, not to resolve a thatch buildup problem.

All About Soil Compaction
Plant roots need air in the soil and good drainage. Without it, the root systems will be shallow and weak. In lawns, signs of compacted soil include poor drainage (puddling), excessive weeds despite the use of good weed controls and poor grass growth despite good maintenance practices. When the soil is compacted, nutrients and water are slow to get to the roots, further weakening the plants. Soil compaction is a problem in high traffic areas or where heavy equipment has been used for construction, grading or even mowing. If you don’t know if your soil is compacted, you can check it out yourself. Take a sharp shovel into the lawn and try to dig out a scoop of turf and the underlying soil. If it isn’t a struggle to sink the shovel in at least half way, you soil isn’t too compacted. Look at the sample you removed. There is probably lots of grass roots that extend 4-6 inches below the surface. You will be able to see spaces in the soil between the particles and it will crumble fairly easily. Now, if you have to jump up and down on the shovel to even get it into the soil, you have a problem. Look at the sample you removed. There probably won’t be a deep, extensive root system for the grass, it will also look fairly solid and will be hard to break up. This soil needs aeration.

All About Aerating
Aeration is best done in late summer or early fall. Depending on your soil type and the traffic on your lawn, it may need aerating every second or third season. Despite all the claims about aerating your lawn by wearing spikes attached to your shoes, mechanical core or plug aerators are the only good way to aerate a lawn. They pull up a core of soil and leave it on the surface of the soil. This does the best job. Core aerators can either be rented or many lawn care companies offer this service. These machines should pull out plugs that are about the size of your little finger. It may sound like too many holes, but the aerator should be run back and forth until there are 20-40 holes in every square foot. The holes allow for air, rain and nutrients to penetrate the soil better. It also gives the roots room to grow. The lawn will look rough for a few weeks, but this is the right amount. Leave the cores on the surface to dissolve over the next few weeks in the rain. The soil in the cores contains millions of microorganisms that help digest thatch naturally, creating a healthier lawn. If we go through a long dry period after core aerating, run the sprinkler to help break down the cores. Avoid using aerators that are simply rollers with spikes. Spikes simply push soil aside when penetrating the soil, adding to the compaction problem. The up side of core aerating is that it encourages deep rooting, increases air in the soil, improves water and nutrient penetration, helps break down thatch and encourages growth of beneficial soil microorganisms. The down side of core aeration is that it also brings up quite a few weed seeds from the soil bank. You may want to consider using a pre-emergent herbicide the following spring to minimize the weed problem.

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