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Extension > Yard and Garden News > March 2011

Thursday, March 31, 2011

Contents: April 1, 2011

In this issue of the Yard and Garden News:

The Good, Bad and Interesting Roles of Nitrogen (N) and Nitrogen Fertilizers in Home Lawn Care - Part 2 of a 3 Part Series on Understanding and Using Home Lawn Fertilizers

Creating a Soil Mix for Blueberries

Wasp Nests and Wasps in Spring

Stowaways in the Garden Center - How to Avoid Hitch Hiking Plant Pathogens

Webbing Clothes Moths

The Good, Bad and Interesting Roles of Nitrogen (N) and Nitrogen Fertilizers in Home Lawn Care - Part 2 of a 3 Part Series on Understanding and Using Home Lawn Fertilizers

Bob Mugaas, UMN Extension Educator

This is the second article of a three part series on Understanding and Using Home Lawn Fertilizers. This article will focus attention on the nutrient nitrogen, why it gets so much emphasis in lawn care and, the roles that it plays in maintaining a healthy, vigorous lawn. As we begin this article, it is important to remember that while other nutrients such as phosphorus and potassium are important for healthy lawns, it is the nutrient nitrogen upon which nearly all general recommendations for home lawn fertilizer applications are based.

Photo 1: Healthy, dense lawn promoted by proper use of N fertilizers.
Bob Mugaas.

Grass plant absorption of nitrogen and its impacts on grass growth
Nitrogen (N) is the mineral element used in the largest quantity by our grass plants. It is absorbed primarily from the soil by the plant's root system. However, nitrogen is not absorbed by the plant in its elemental form (N). The two most commonly absorbed forms are the nitrate ion (NO3-) in which N is combined with oxygen(O) and contains a single negative charge on the molecule and the ammonium ion (NH4+) in which N is combined with hydrogen(H) and contains a single positive charge on the molecule. Of these two forms, the nitrate form is absorbed in the largest quantity by the plant root system.

Once N is inside the plant it becomes an integral part of many other plant molecules and processes. Among those various roles, one of its most important is in stimulating shoot growth which aids in spring green up and helps promote recovery from injury and environmental stresses. Interestingly, shoot growth is often stimulated at the expense of root growth. Thus, lower, more modest use of nitrogen fertilizer, especially in the spring of the year, is actually healthier for the plant as shoot growth will be more in balance with root growth. In turn, the plant will be in a healthier condition and better able to survive the more stressful summer growing conditions. See Picture 1.

Nitrogen also plays an important role in the production of the green pigment known as chlorophyll which in turn is responsible for producing the green color in grass. This is why you often find yourself mowing more frequently and observing a deeper green color to your grass following a nitrogen fertilizer application. After a few days following a nitrogen fertilizer application, any lawn areas that were skipped or missed will also be very evident. Non-fertilized areas or skips will be distinctly lighter green compared to those areas receiving fertilizer. Depending on the degree of misapplication, it may be quite embarrassing as well.

Photo 2. Bob Mugaas.

Nitrogen needs can vary by grass species
While nitrogen is used by grass plants in larger quantities than other nutrients, there is some variability as to how much is needed among the different lawn grass species. For example, Kentucky bluegrass and perennial ryegrass require larger amounts of N than the fine fescues to remain healthy and vigorous. This is one of the main reasons why the fine fescues are the plant materials of choice when it comes to implementing low input lawn care programs or where no-mow plantings are desired. Under otherwise favorable growing conditions, fine fescues can get by with as little as 1# or even less of actual N per 1000 ft2 per year.

Picture 2 illustrates the difference in texture between Kentucky bluegrass and the fine fescues. If your lawn is dominated by the finer textured fine fescue grasses, you can likely reduce the amount of nitrogen applied as noted above and still expect to maintain acceptable or better lawn quality.

Why the need for supplemental nitrogen

While grass plants are moderate to high users of nitrogen depending on species, it is not the only fate of nitrogen in our soils. There are two other soil related processes that play an important role in how much nitrogen is available for plant growth during the growing season. First, the nitrate form of nitrogen can easily be carried along with the vertical movement of water down through the soil. This is the process known as leaching. It effectively moves nitrate nitrogen beyond the root systems of our grass plants and now has the potential to continue downward through the soil and potentially pollute ground water supplies. Second, nitrate nitrogen can also be converted to nitrogen gas (N2) through a process known as denitrification. This most commonly occurs during warm, wet soil conditions.

The end result of these three major soil nitrogen losses along with losses from a few other minor soil processes is that our soils often lack the necessary amount of nitrogen needed to support healthy, vigorous grass growth at all times during the growing season. Hence, there is often a need to supplement our soils with some additional nitrogen containing fertilizer in order to sustain a healthy turfgrass system throughout the year.

Natural nitrogen inputs to a lawn
Before delving into nitrogen fertilizers, it is important to remember that fertilizer is not the only means by which our lawns receive supplemental N inputs. For example, as organic matter is continually being decomposed by the soil microbes, nutrients, including plant available nitrogen, are released back to the soil for use by the grass plants or perhaps reused (and rereleased) by other microorganisms. When soil test results indicate organic matter levels of 3.1% or higher, about 0.5 pounds (or slightly more) of actual nitrogen per 1000 ft2 can be returned to the lawn annually.

Grass clippings generated during regular mowing and left on the lawn over the entire season will supplement the lawn with the equivalent of about one complete fertilizer application annually or about 1 pound of actual N per 1000 ft2 per year. Under very low maintenance situations, including no irrigation, these natural supplies of N may be sufficient to sustain the turfgrass. However, some (0.5 - 1.0 #N per 1000 ft2) supplemental nitrogen will usually be necessary to sustain an acceptable turfgrass stand having at least medium green color and sufficient density to completely cover the soil surface.

Photo 3: Necrotic ringspot disease on Kentucky bluegrass.
Bob Mugaas.

When there is too much nitrogen
Over abundance or deficiency of available nitrogen are both problematic for grass plants. Excessive levels of nitrogen:
1) stimulate rapid shoot growth while slowing down root growth and increasing the need for more frequent mowing; 2) deplete the plant's carbohydrate reserves more rapidly, which in turn can result in less stress tolerance and slower recovery from any injury to the plant; 3) result in thinner, more succulent leaf tissue, which increases moisture loss and therefore creates a greater need for water; 4) can predispose the plant to greater insect and disease problems; (Picture 3 illustrates Necrotic Ring Spot disease on Kentucky bluegrass; a disease often associated with excessive nitrogen fertilizer applications.); 5) contribute to more rapid and excessive thatch development; 6) leach through the soil beyond the root system potentially polluting ground water resources when not used by the grass plant.

Photo 4: Crown rust on Kentucky bluegrass.
Bob Mugaas.

When there isn't enough nitrogen
On the other hand, a deficiency of available nitrogen results in: 1) slower grass plant growth; 2) paler green color thus decreasing photosynthetic capacity and therefore food production; 3) slower recovery from injury thus decreasing traffic and wear tolerance; 4) increased susceptibility to crown rust, red thread and dollar spot diseases; (Picture 4 illustrates crown rust on Kentucky bluegrass.); 5) decreased tolerance to environmental stresses such as drought conditions and/or high temperatures; 6) decreased lawn density thus opening the door for increased weed invasion and decreased ability to both reduce the amount of runoff as well as slow the velocity of runoff water from the site.

A brief overview of forms and sources of N fertilizer:

While a University of Minnesota soil test report can be very helpful in determining what your lawn needs, it doesn't measure the amount of nitrogen actually in the soil as part of a standard soil test. Because of the various ongoing and ever changing nitrogen losses described above, a nitrogen value won't be very helpful. In other words, a measured N level today will not likely be the same a week from now or even in a few days from now depending on weather conditions, grass growth and cultural practices being carried out. Nonetheless, the soil test report does consider the various forms of nitrogen losses and natural inputs. Thus, a soil test report will suggest a nitrogen application program based on soil test results and information provided by the user at the time the soil sample was submitted.

Quick release vs. slow release N sources. There are many sources and formulations of nitrogen found in lawn fertilizers. However, those sources can be broken up in to slow release sources (water insoluble nitrogen) or quick release sources (water soluble nitrogen). In the latter, the fertilizer is readily soluble in water and, once dissolved, the nitrogen is immediately available for grass plants to take up and use. Common examples of quick release sources include ammonium sulfate, ammoniacal nitrogen and urea. As a result, grass plants green up very quickly and grow vigorously (sometimes too much so) for a relatively short period of time following an application of quick release nitrogen.

An over application of quick release nitrogen and the accompanying excessively lush grass growth also increases the need for more moisture and more frequent mowing and can make the lawn more susceptible to certain diseases, insect problems and environmental stresses. In order to avoid this situation, it is highly recommended that quick release nitrogen fertilizer be applied at no more than one pound of actual N per 1000 2 of lawn per application.

In contrast to the rapid green up and rapid growth rate resulting from quick release sources of N, slow release sources result in a slower rate of green-up and growth. In addition, they can provide a longer period of sustained nitrogen release to the lawn. This extends the length of time the grass exhibits a healthy green color and sustains a more uniform, but slower growth rate. For these two reasons, it is also likely that the time between fertilizer applications will be lengthened and may even reduce the need for nitrogen fertilizer applied in a growing season.

Organic, synthetic organic and inorganic sources. Slow release sources can originate from natural sources such as those produced from plant or animal processing byproducts or be synthetically manufactured. The former are known as natural organic nitrogen sources while the latter are known as synthetic organic nitrogen sources. The usage of the word organic in this case simply means that the products contain the element carbon (C) in their molecular structure(s). When carbon is absent from the fertilizer molecule, they are known as inorganic fertilizers.

Photo 5: Well-planned shoreline landscape including a healthy lawn area.
Brad Pedersen.

Most of the quick release fertilizers are inorganic while slow release fertilizers are mostly organic in origin. The one notable exception is the fertilizer material known as urea. It does contain carbon in its molecular structure and therefore is considered an organic material but it reacts like a quick release nitrogen source when used in a lawn fertilizer. Urea is often the starting point for many of our synthetic organic nitrogen sources used in lawn fertilizers. These products are produced by either further chemically reacting the urea such as in the case of producing methylene urea or coating the urea such as in the case of sulfur coated or polymer coated urea. Both methylene urea and sulfur coated urea are commonly available in homeowner lawn fertilizers.

Slow release fertilizers and water quality impacts. There are also positive water quality implications when using slow release sources of N. Lawns, landscape plantings and shorelines can be compatible partners in helping to protect water resources. See Picture 5. Using slow release fertilizers can be an important tool in managing our lawns to protect water quality. Since smaller amounts of N are released over a longer period of time, grass plants have a greater chance of taking up and using the N before it potentially leaches through the soil and has a chance to contaminate ground and/or surface water resources. Because the nitrogen is released more slowly, slightly higher rates of fertilizer can be applied. As always however, be sure to follow the directions on the fertilizer container for proper spreader settings and application rates.

For more information

For a comprehensive listing of nitrogen sources and a more thorough discussion on when and how to correctly apply lawn fertilizers including calibrating a lawn fertilizer spreader for accurate application, see the Home Lawn Care Chapter on Understanding and Using Home Lawn Fertilizers in the Sustainable Urban Landscape Information Series website.

In summary
Nitrogen is an important element in sustaining normal, healthy grass plant growth and aiding recovery from injury. Used wisely and applied at appropriate times of the year, the likelihood of creating adverse effects to the lawn or the surrounding environment will be minimal.

Creating a Soil Mix for Blueberries

Karl Foord, UMN Extension Educator

Exhibit 1, click to enlarge.

Plants have a soil pH range within which they function well (Exhibit 1); however outside this range plants can show signs of stress. This stress is often in the form of a pH induced nutrient deficiency. In blueberries an out of the range high soil pH will induce an iron deficiency (Photos 2 & 3). To get a sense of pH, a number of commonly encountered materials and their respective pH values are listed in Exhibit 1.

Traditional blueberry soils are sandy with low organic matter and pH of 4.5 to 5. In addition blueberries plants do not tolerate waterlogged or droughty soils. Unfortunately, my soil is a clay loam soil whose two soil tests results indicated pH values of 6.8 and 7.2. In addition the soil was compacted during house construction so the depth of the hole for planting needs to be deeper than the compaction zone, or the blueberries should be planted on raised beds or both. If the planting hole is filled with water and takes more than four hours to drain, the hole is not deeper than the compaction zone.

Photo 2: Iron-deficient plants have chlorotic leaves with green veins.

Symptoms develop first on the young leaves at the shoot tips.  It will be a significant challenge to modify the pH 7 clay soil to the point where it will be amenable to blueberries. Clay soils tend to resist changes in pH and dramatic changes will be necessary to achieve a soil pH of pH 4.5. A pH of 4 is not three points away from 7 it is 1000 because the pH scale is logarithmic. Rather than attempt this change, the plan is to abandon efforts to change the soil and start over with a new soil. The soil mix proposed is as follows:

Proposed blueberry soil mix:

50% Sphagnum peat moss - (pH of 3.6 to 4.2)
10% Original soil - (pH 7 clay loam)
10% Compost - (likely pH @ 7)
10% Sand Inexpensive, untreated sand used primarily in construction
20% Perlite

Logic of proposed mix
The peat moss will provide organic matter and be the source of low pH for the mix. It will have to overcome the high pH characteristics of both the compost and soil components of the mix. Perlite and sand will add drainage qualities to the soil mix. The only way to truly know if the soil mix is at the proper pH is to have it tested.

High pH characteristics of compost
Although tree leaves tend to be acidic (Exhibit 4), compost made from tree leaves and most yard wastes will likely be alkaline (pH between 7.0 and 8.0) due to changes in compost components during the composting process. Most manures also have a liming-effect (increase in soil pH) as the pH of cattle and sheep dung is typically >7.0.

Planting and maintenance

To be extra cautious about pH and drainage, remove the soil creating an 18 inch deep by 3 foot wide bed. Fill the ditch and add enough mix to create a 12 inch high raised bed. Space the plants 3 feet apart to create a hedge. Because the blueberry planting is a low pH island in a high pH field with environmental conditions that will tend to increase the pH, fertilize with ammonium sulfate or urea to help maintain the proper pH. Amend any mulch you use with a small amount of elemental fertilizer. See the UMN Extension website for dosage recommendations.

Exhibit 4: click to enlarge.

Key summary statement

This particular mix will work as will many other combinations. Given the tendency for many of the soils in Minnesota to be alkaline (high pH), this method may also prove valuable for planting of other acid loving crops such as rhododendrons and azaleas. It bears repeating that the main thing to do is get your soil mix tested and be sure that it is at the right pH for blueberry and other acid loving plants.

Wasp Nests and Wasps in Spring

Jeffrey Hahn, Asst. UMN Extension Entomologist

Photo 1: Don't worry about wasps in this nest in early spring.
Jeff Hahn.

As our attention finally turns to spring, there could be some loose ends to tie up from last fall. For people that experienced wasp nests in their home, they may be wondering what they should do with any wasps that may remain in them. Fortunately, there are no longer any wasps alive in those nests.

Late last summer, new queens were produced. After mating, they left the nest, and flew off to eventually find sheltered sites in which to overwinter. Meanwhile back at the nest, the old queen and workers continued with their daily routines until freezing weather killed them. Newly mated queens do not return to their old nests but instead will construct their own nests when spring begins.

So what does that mean for nests in and around your home this spring. You can largely ignore them. One exception to this would be if you experienced a wasp nest in a wall void or somewhere within the structure of your home, such that you could not see the nest but you can see wasps flying back and forth from an opening. In this case, you should seal up those openings. The reason is that while the old nest is not reused, a new nest could be built in the same space. Early spring is a good time to seal those openings before wasp queens are active and begin building new nests.

Photo 2: Paper wasps overwinter gregariously.
Jeff Hahn.

There are have also been reports of wasps indoors during mild late winter days. There has been concern that there is a wasp nest in the home and the warm temperatures are 'waking' them up. While most wasp queens will overwinter on their own, paper wasp (Polistes spp.) queens seek shelter gregariously, i.e. in nonsocial groups. It is not uncommon to see a dozen or more paper wasps but fortunately, this is not an indication of a nest. If you encounter this situation, just open the window and let them fly out or take a fly swatter or rolled up newspaper and dispatch them.

Stowaways in the Garden Center - How to Avoid Hitch Hiking Plant Pathogens

Michelle Grabowski, UMN Extension Educator

Photo 1: Container production in the western United States.
M.Grabowski, UMN Extension.

Many Minnesota gardeners are anxiously awaiting the melting of snow and the warming of soil. As you decide on what new plants to add to your garden this year, consider carefully where those plants are coming from and if there is a risk that they might not be coming alone.

As the world becomes more internationally connected, common garden plants are being shipped across the country and in some cases across the ocean. This is not a problem unless there are plant pathogens hitching a ride on these garden plants. In 2009, tomato transplants from the south eastern United States were shipped to garden centers from Maine to Ohio. Unfortunately these tomato transplants were infected with the pathogen responsible for causing a devastating disease of tomato and potato known as late blight. The resulting disease epidemic spread from home gardens to commercial tomato and potato fields; from the east coast through Wisconsin.

Devastating diseases such as White Pine Blister Rust and Chestnut Blight are believed to have been brought into the United States on infected nursery stock. Other invasive pest problems, like Dutch Elm Disease and Emerald Ash Borer, are believed to have been moved across the U.S. on infected firewood. As responsible gardeners and plant lovers, it is important to choose garden plants wisely.

Photo 2: Minnesota grown basil transplants.
M.Grabowski UMN Extension .

Where are pathogen stowaways hitching a ride?

  • Nursery plants
  • Seeds
  • Potting soil
  • Wood or other plant products

How can you avoid stowaway pathogens?
Buy locally grown plants

  • Locally grown plants have local plant disease problems, plants shipped from distant states or other countries may contain pathogens from those areas that were not previously found in MN.
  • Vegetable and annual flower transplants grown from seed in MN greenhouses are grown March through June. Very few pathogens are active this time of year. The same plants grown in warmer climates may be exposed to many different fungi and bacteria before being shipped north.

Photo 3: New seedling trays, provide seedlings a clean start in life.
M.Grabowski, UMN Extension.


  • Shop for vegetable and flower transplants at local farmers markets.
  • Ask at the garden center where transplants are coming from.
  • Read plant labels and choose plants from local nurseries.

Start plants from seed

  • Only a few pathogens can travel on seed. Many more can be carried in infected potted plants on roots, stems, leaves or in potting soil.


  • Purchase seed from a reputable producer to avoid problems with contaminated seed.
  • Purchase certified disease free seed when possible. This is more common with fleshy 'seed' like potatoes or bulbs than with true seeds.
  • Plant seeds in new potting soil with new or sterilized pots and trays. For more information about starting plants from seeds, read the UMN Extension publication 'Starting Seeds Indoors'

Select container grown plants wisely

  • Potted plants can harbor plant pathogens on roots, leaves, stems, flowers or in potting soil.

Photo 4: Healthy plants from a reputable nursery are a good place to start a healthy garden.
M.Grabowski, UMN Extension.


  • Purchase plants from a reputable nursery.
  • Choose locally grown plants whenever possible.
  • Purchase certified disease free plants if possible. This is common with some plants like certified virus free roses, raspberries and hostas, but not all plants have a grower certification process.
  • Inspect all plants prior to purchase. Reject any plants that have dark, soft or sunken spots on roots, leaves or stems. Reject any plants that have diseased neighbors in the garden center as disease spreads easily from one potted plant to another.
  • Report diseased plants to the garden center management.

Do not move firewood

  • Invasive insects and pathogens can be carried on wood from infected trees.


  • Burn firewood in the same area from which the tree was harvested.

Learn more about the plant diseases in your garden and report suspected invasive pests

Webbing Clothes Moths

Jeffrey Hahn, Asst. UMN Extension Entomologist

Photo 1: Webbing clothes moth adult.
Jeff Hahn.

The most common small moths found indoors, e.g. Indianmeal moths, are usually associated with stored foods. However, occasionally you may encounter small moths that attack fabric. The most common species is the webbing clothes moth. The adult webbing clothes moth has buff colored wings with no spots or markings on them. The wings are folded behind their back when at rest and the insect measures about 1/4 to 1/3 inches long. Particularly characteristic is the mop of reddish brown hairs on its head. Webbing cloth moth adults avoid light and generally seek out dark areas of rooms.

The larvae are whitish with dark colored heads and are no larger than ½ inch long. They feed on wool, fur, feathers hair, and other materials of animal origin. They are not interested in plant material, like cotton and generally do not attack synthetic material unless it is blended with wool or is stained, e.g. with sweat or food. Webbing clothes moth larvae graze on the surface of the material, creating irregular holes in the fabric. They also create silken tubes or mats on the fabric as they feed which helps to identify the damage.

Prevention is the best control to avoid webbing clothes moths. You can do this through regular vacuuming including carpets and rugs, along baseboards, under furniture and in closets as well as removing scraps and remnants of wool, fur, and similar materials. Regularly inspect susceptible clothes, like wool sweaters and suits, wool rugs, animal mounts, and other material for signs of infestation; the sooner an infestation is discovered, the sooner it can be controlled.

Photo 2: Webbing clothes moth damage, adult and larva.

When putting susceptible clothes away for the summer be sure they are dry cleaned or laundered. It best to place them in airtight containers, such as a plastic sweater box. Wood chests can also work if the top fits tightly. For added protection, you can place mothballs or crystals (naphthalene) with the clothes in the storage containers. However, keep in mind the smell of naphthalene may be challenging to remove from clothing. Dry clean clothing again before wearing to help remove any odor. Remember that naphthalene is not a repellant but instead uses a concentration of vapors to kill insects. Placing moth balls or crystals loose in a closet or similar areas will not prevent webbing clothes moths. Cedar chips, although popular as an insect repellent, do not effectively deter webbing clothes moths.

If you find an infestation of webbing clothes moths, either remove and throw away the source of the infestation or have it laundered or dry cleaned. Be sure to vacuum or other wise clean up the immediate area to remove any potential additional sources of infestation. Severe webbing clothes moth infestations may require the service of a pest management service.


Begin to move protective cover from bulb beds, non-hardy roses, and perennials in stages as mulch and soil thaw. Don't rush to uncover tender plants, though! Mulch helps prevent the plants from coming out of dormancy too early, when damaging cold is still a possibility. Rose canes will be okay as long as temperature hover around twenty degrees, but most flowering perennials will die back when it's that cold.

April showers bring May flowers... keep an eye on the spring warming trends at University of Minnesota Climatology Working Group website!

Have your soil tested if you plan to start a new flower or vegetable garden-- or if your existing garden is not productive and performing well, even though it gets plenty of sunlight an water. The University of Minnesota Soil Testing Lab will analyze the soil and recommend fertilizer and other soil amendments, based on reliable data.

Yard and Garden News Editor: Karl Foord
Technical Editor: Bridget Barton

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