ISSUE 3    May 18, 2006

EXCESSIVE THATCH IN LAWNS

Thatch, the layer of organic matter between grass foliage and underlying soil, is normally healthy for a lawn, when it is about ½ inch thick. Excessive thatch, however, can be detrimental. When too thick, thatch prevents grass roots from reaching the underlying soil, where the plants obtain necessary moisture and nutrients. As a result, affected grass may wilt during hot, dry periods. The added stress further predisposes the grass to diseases. Causes of excessive thatch include compacted, poorly-drained soils, over-use of pesticides that eliminate populations of beneficial insects and microorganisms that normally break thatch down, and less-than-ideal cultural practices, such as waiting too long between mowing events. (Contrary to myth, bagging clippings does not necessarily reduce thatch. In fact, studies consistently show that allowing the clippings to remain on the grass is usually beneficial to the lawn, if the clippings don’t fall in clumps and if the grass does not suffer from a foliar disease.)

De-thatching is recommended when thatch layers exceed ½ inch. Power raking is one method that can be used to effectively de-thatch a lawn. This procedure should be done during a time that favors the growth and recovery of the grass (when the grass is green and actively growing), such as during May or mid-August in North Dakota. Avoid power-raking in mid-summer, during hot, dry periods, since the procedure can be very stressful to the grass. The cool-season grasses that are predominantly grown in North Dakota will struggle to recover after the stressful procedure under typical summer conditions. Power-raking in early spring is also not recommended, since the grass is not yet actively growing and so cannot compete as well against the spring weeds which can get a foot-hold in the open spots left after power-raking. Over-seeding is recommended if more than half of the desired lawn species is removed.

A list of NDSU extension bulletins related to lawns can be found at http://www.ext.nodak.edu/extpubs

 

SPRUCE TREE PLANTING TIPS

Spruce trees have shallow root systems, and they grow poorly in wet, inadequately drained sites. When establishing spruce trees, consider the following recommendations:

1. Choose a well-drained site.

2. Provide ample room between spruce trees, to allow for good aeration between trees. Plan enough space for when trees are full-grown, to avoid having to thin the trees later on. Closely planted spruce trees are not recommended, since they create an environment that can favor development of diseases such as needle casts or Cytospora canker.

3. Water adequately, but avoid over-watering, since spruce do not like wet soil for prolonged periods.

4. Fertilize judiciously. Avoid fertilizer applications in fall, to prevent lush growth which can be more susceptible to winter injury.

5. To further help minimize winter injury, provide water during hot, dry periods, especially during late summer to early fall, to allow trees to prepare for winter properly

6. Choose disease-resistant varieties, when available.

7. If planting many trees, try to avoid planting all trees of the same species (for example, avoid planting all blue spruce, or all black hills spruce).

8. For weed management:

a. If weed pressure is low, hoe or pull weeds out by hand.

b. Avoid chemical herbicides for weed control. Due to the shallow root systems of spruce trees, they are particularly sensitive to many chemical herbicides such as caseron, dicamba, and others. Even if a herbicide is considered safe to use under spruce, using it may be risky for spruce trees.

c. Established spruce trees are susceptible to root injury by tillage or cultivation that is deeper than previous cultivation or tillage events. Completely avoiding cultivation or tillage, especially under older trees, is recommended.

d. Avoid using string mowers (a.k.a. weed whackers) near the trunks of trees. These devices can cause injury known as girdling, where protective bark is removed for part or all of the circumference of the trunk. Generally, girdling of about 2/3 (more or less) of a tree diameter will likely lead to tree death.

e. Avoid the use of landscape cloth barriers or plastic mulch – they may not allow proper oxygen levels to reach the shallow roots. Organic, loose mulches, such as pine needles, bark, dried grass clippings, straw, and others are better choices for mulches. The mulch, laid about 2-6 inches thick, should reduce or eliminate weed problems under trees, and it can also help hold in moisture during dry periods. When adding mulch, be careful not to lay it too thickly. Also, be sure it is not in direct contact with the main stem – this can create an environment favored by rodents to chew around the base of the stem and girdle the tree.

f. Nearby construction and trenching can cause serious injury to tree roots, so it is best to completely avoid any construction or trenching near trees.

g. Paving over tree root systems can also cause serious injury, by limiting the amount of oxygen that reaches the roots.

For a list of NDSU Extension publications on trees, go to http://www.ext.nodak.edu/extpubs

Other publications that offer more information on tree selection and care are below:

Tree Selection Guide for Western North Dakota http://www.ext.nodak.edu/county/stark/tree.htm

N.D. Tree Handbook http://www.ag.ndsu.edu/trees/handbook.htm

Kasia Kinzer
NDSU Plant Diagnostic Lab
diaglab@ndsuext.nodak.edu
http://www.ag.ndsu.nodak.edu/diaglab
701-231-7854

 

 

 


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