Skip to Main navigation Skip to Left navigation Skip to Main content Skip to Footer

University of Minnesota Extension

Extension > Garden > SULIS > Maintenance > Sustainable Herbaceous Plant Maintenance > Mulching and Watering

Print Icon Email Icon Share Icon

Mulching and Watering

The Eco-Yard of Hennepin County
Environmental Services,
Brooklyn Center, MN
Mulching is a long-time practice of gardeners to prevent loss of moisture and to control weeds. It is also increasing in favor as municipalities restrict the use of water for lawns and landscapes. Mulch is a layer of organic or inorganic material placed over the root zone of a plant to benefit the roots and the soil. Organic materials may include wood chips, bark, pine needles, straw, leaves, or grass clippings. These materials will eventually decompose, adding organic matter to the soil and need to be supplemented or replaced on a regular basis. Inorganic materials typically do not decompose, need replacement only occasionally, are often more expensive, and include rock or pebbles, landscape fabric, newspaper, plastic or even shredded rubber tires for playground areas.

Why use mulch?

  • To conserve moisture in the plant root zone;
  • To prevent weed growth;
  • To stabilize soil temperatures, reducing plant injury from the heaving of plants during freeze/thaw periods and allow for gradual warming of the soil in the spring;
  • To prevent the spread of soil-borne diseases;
  • Increase soil fertility through decomposition of mulches;
  • To increase water infiltration and prevent soil compaction;
  • To improve the appearance of landscapes.

Application of Mulch  Summer mulch should be applied to soil in late May / early June after the soil has warmed. Winter mulch should be used to cover plants in the fall after the ground is frozen. Mulch depths range from 2-3 inches in herbaceous and vegetable beds to 4-6 inches for winter protection. Thicker depths will cause moisture to be retained and a lack of oxygen to the roots. This anaerobic environment will cause such problems as root rot, a condition caused by root suffocation. This lack of oxygen will cause symptoms such as yellow foliage, lack of growth, dieback, small leaves, etc. Thick mulch can also become a haven for small, burrowing animals who feed on plant stems.

Mulches such as wood chips, straw, sawdust, and bark have high carbon-nitrogen ratios and thus additional nitrogen should be added to the soil prior to applying the mulch. If additional nitrogen is not supplied, the microorganisms responsible for decomposition will use the available nitrogen, causing the plants to show symptoms of nitrogen deficiency.

Mulching for Weed Control
Mulch is often used to control weeds. This may be one of the reasons people tend to apply mulch too thickly. Mulch that is 2-4 inches deep is sufficient for blocking out light that causes weed seeds to germinate. It will also prevent soil-borne bacterial and fungi from splashing onto leaves during rainstorms or watering.

Perennial beds are best protected with 2-3 inches of mulch such as shredded leaves, shredded bark or woodchips. Annuals benefit most from 1-2 inches of organic mulch that will break down quickly and is tilled into the soil each year. This adds considerable organic matter to the soil.

Even thought they are often planted to control weeds, ground covers should be mulched after planting to prevent weed infestation and improve establishment, survival and growth. The spaces between the plants can be filled with approximately two - four inches of mulch such as compost, well-rotted manure, woodchips or shredded bark if planted on a slope. Remember to apply nitrogen prior to using wood products to prevent competition for nitrogen between the plants and bacteria that decompose the wood products. Another mulch option is to plant non-aggressive annual flowers between the plants to help shade out the weeds without competing with the ground cover. Be sure to provide a sufficient amount of water for both types of plants.

Mulching for Winter Protection
It is a common misconception that covering plants with mulch in the fall keeps them from freezing. While mulch does help to insulate the soil, the soil usually freezes. However, the mulch helps this change to be gradual. Mulch also reduces the heaving of the plant crown caused by the freeze / thaw cycle. The same is true with warm temperatures. Mulch helps to maintain cool soil temperatures in the spring, gradually warming up, preventing the damage that can occur to plants when they emerge before temperatures are warm enough to sustain them. An application of mulch in late spring after excess moisture from spring thaws has evaporated will keep the soil as much as 10° cooler in the summer heat. Black plastic can be used to increase soil temperature more rapidly for heat-loving plants, but it restricts the flow of air and oxygen.

Perennials that are marginally hardy require winter protection in the form of mulch such as shredded leaves, straw, pine needles, etc. Mulch should be applied in late fall when there is some frost on the ground and removed in the spring when soil temperatures have warmed and active growth on the plants is noticeable. Waiting too long to remove the mulch in the spring may result in mold growth and damage to the plants. Usually mid-April is a good time to remove mulch in Minnesota, but gardeners should be prepared to cover plants if a late freeze is predicted. Keeping a supply of mulch on hand for such an occasion is recommended.

Mulching for Moisture Control
Mulch is an excellent means by which to conserve moisture in soil. Mulching plants in the fall helps to hold in much-needed moisture and prevents tissue burn and desiccation during the winter. For more information about annuals that are drought tolerant, and suitable for shade and sun, visit these sites:
Drought-Tolerant Annuals
Annuals for Sun
Annuals for Shade

Organic Mulches
Composted animal manure - horse, cow, sheep, chicken
Grass clippings
Shredded leaves
Composted municipal sludge
By products of the Food Industry - peanut and rice hulls, cocoa bean shells, crushed corn cobs
Pine needles
Straw and hay
Wood products - chips, bark, shredded, sawdust, waste wood

Synthetic Mulches
Plastic - black, clear
Rock and pebbles
Shredded rubber tires
Landscape fabric / Geofabric
Landscape paper

Note that while plastic prevents weed growth, it is not a good choice for long-term mulch as it does not allow water to reach the plant roots. Landscape fabric or geofabric is a better choice for long-term weed prevention and is best used in combination with an organic mulch such as woodchips.

Watering Your Landscape
There are many variables when it comes to watering your yard and gardens - the weather, the amount of sun the area receives, sprinkler vs. soaker hose, humidity level and, most importantly, the soil and plant material you are watering. There is a lot of new information about planting sustainable landscapes including xeriscaping - planting drought-tolerant plants in areas of a landscape that are difficult to water. "Right plant, right place" is an important phrase to remember when considering planting in an area where a water spigot is not available or, on a larger scale, planting in a part of the country where water usage is restricted.

Mulching How to Determine Frequency and Rate of Watering
Plant type and soil type are the two most important factors when determining how much to water and how often. Most people just look at plants to see if they have begun to wilt - that indicates the plants need water, right? Wrong. Plants that have begun to wilt have already suffered water stress - a condition that should be avoided because it can lead to a weakened plant that cannot defend itself against insects, diseases and the elements. Sometimes plants wilt in the heat of midday sun. In most cases, this is not a concern and watering at this point may result in more than half the moisture evaporating from the soil surface. However, if you observe plantings wilting in the cooler conditions of early morning or evening, chances are they are in need of water.

As a rule of thumb, water infrequently and give plants a deep, thorough soaking. Frequent, shallow watering causes plants to produce shallow roots that cannot survive the heat and dry conditions of mid-summer months. Watering deeply and infrequently causes plant roots to grow deeply into the soil in search of the water, resulting in deeply rooted, more drought resistant plants.

Over-watering is as detrimental to plant health as under-watering. Typical gardens require about one inch of water per week (rainfall + irrigation), depending on the plant material and soil type. Many native plants and prairie plants common to Minnesota landscapes have evolved over centuries to thrive in dry soil typical of the native prairies of our state. Over-watering these plants creates conditions that are detrimental to plant health and invites diseases such as root rot and Verticillium. The root systems of plants require oxygen in addition to water. Over-watering fills the pores between soil particles, creating conditions that are anaerobic - without air - and literally suffocating the roots.

Letting the soil dry out somewhat between watering is one way to avoid this situation. An easier method of determining whether plants need water is to actually feel the top 6-8 inches of soil. Roots of most herbaceous plants reside in the top 8-12 inches of soil. Clay soil will hold more water, and sandy soil will release water more rapidly. In both cases, organic matter should be added to the soil in order to improve its structure and increase the soil's water-holding capacity for sand and aeration for clay.

Water Quality and Its Effects on Plants
Landscapes are irrigated from a variety of water sources depending on the site. Some homeowners have a well; others are on municipal water. Some people collect rainwater and use it for watering gardens and potted plants. Below are various types of water used to irrigate plant material:

Rainwater that is unpolluted is one of the best choices for irrigation because it contains few contaminants. However, collecting rainwater and methods of utilization can be difficult. Acid rain is caused by contaminants such as nitric oxide and sulfuric acid that are industry pollutants. These pollutants are carried into the atmosphere by the wind where they are mixed with water vapor in clouds. The result is acid rain. Acid rain lowers the pH in bodies of water and can affect the pH of soils. At this time, acid rain not a concern for Minnesota except in the Northeastern part of the state.

Mulching Municipal water and well water. Municipal water is often treated with salts and chemicals such as chlorine and fluoride that can result in excess salts in the soil, resulting in injuries to plant tissue and changes in the levels of available nutrients. Hard water contains high levels of calcium and magnesium, two elements which, when combined with soap, reduce the suds and can leave a residue on fixtures. When water is "softened", sodium replaces the calcium and magnesium, allowing soap to suds up and clean better.

While calcium and magnesium are necessary plant nutrients, only a small amount of sodium is required by plants. Occasional leaching with rainwater or unsoftened water helps lower sodium concentration in soil. Providing good drainage for plants will also prevent sodium from building up in soil. Plants in containers may be re-potted to eliminate sodium build-up as well.

Chlorine is found naturally in small amounts and plants will benefit from trace amounts of this element in water. Chlorine is also added to water systems as a disinfectant and has proven to be toxic to plant life when used in excess. Chlorine can be removed from water by gentle heating, or by allowing treated water to stand in an open container overnight. The chlorine changes to a gas and is released from the water.

Fluorine may be found naturally in water sources or introduced via fertilizers and industrial pollutants. It is also added to municipal water sources to prevent tooth decay. In plants, excess fluoride may cause burning of plant tissue, especially the leaf tips. The fluoride moves through the leaf tissue where it becomes concentrated in the leaf tips, causing necrosis. This condition is most noticeable in plants with parallel venation that are regularly watered with tap water such as the dragon tree (Dracaena marginata) and spider plants (Chlorophytum comosum). A solution is to incorporate limestone in the soil at a rate of 2 teaspoons per six-inch pot. This will raise the pH of the soil enough to convert the fluoride to a form that is unavailable to plants.

Distilled water is creating by vaporizing water to steam and cooling it. It is relatively contaminant and salt free, but is expensive to use for large areas such as landscapes. Distilled water has also been shown to be damaging to some plants.

Reverse osmosis (R.O.) is a process that removes chemicals from water by running it through a series of filters. Once the equipment is in purchased, R.O. water is inexpensive and a good water source for plant irrigation.

For additional information about water quality, visit On Gardening/Water Quality And Indoor Plants In The Winter by Dr. Robert Nuss, Penn State.

Soluble salts and pH levels are two ways in which water quality is measured. Understanding the effects of excess salt and the importance of water pH on soil quality enables gardeners and homeowners to make good decisions about water sources and the use of fertilizers, de-icing salts, and managing run-off.

Salt or sodium (Na+) is one of the most detrimental elements to plants. Water that is high in sodium is toxic to plant tissue. It can alter the structure of soil and reduce its water-absorption capabilities. For example, when clay particles absorb sodium, the result is a "slick" soil that when moistened will seal the surface of the soil.

The pH level measures the acidity and alkalinity of a water sample by measuring the hydrogen ion (H+) concentration. Pure water has a pH of 7.0. Water with a pH below 7.0 is considered acidic; water with a pH above 7.0 is considered alkaline. Soil pH is affected by the type of soil parent materials, rainfall, irrigation, leaching, organic matter content, and fertilizers. Soil pH affects the availability of various compounds found in soils including nutrients. In some cases, these concentrations can become toxic to plant life. In addition, the effective and safe use of certain herbicides can be affected by soil pH levels. Acidic soils can display such problems as toxicity of various elements, deficiency of calcium and magnesium, poor bacterial growth, reduced nitrogen transformed into available nitrate sources, and reduced available phosphorus. Alkaline soils can feature deficiencies of elements such as iron, manganese, and zinc, an excessive level of salts, and reduced available phosphorus. For more information on altering soil pH, visit Modifying Soil pH.


Clemson University, "Soil Acidity and Liming (Overview)", Soil Acidity and Liming - Part 2, Internet In-service Training, prepared by Charles C. Mitchell, Auburn University.

Iowa State University Extension, "Watering 101 - A Short Course", Cindy Haynes and Elaine Edwards, March 12, 2001.

Kansas City State University Research and Extension, Wyandotte County, "Annuals for Sun", Pat Lawson.

Kansas City State University Research and Extension, Wyandotte County, "Annuals for Shade", Pat Lawson.

Kansas City State University Research and Extension, Wyandotte County, "Drought-Tolerant Annuals", Pat Lawson.

New Mexico State University Cooperative Extension Service, "Irrigation Water Classification Systems", guide A-116, C. A. Glover. Updated February 2001.

Penn State University, graphic, "Mulching Plants and Shrubs for Growth and Health" Penn Pointers.

Penn State University, "On Gardening / Water Quality and Indoor Plants In the Winter", Dr. Robert Nuss, January 12, 1998.

Texas A&M University Agricultural Extension Service, online information, "Water and Water Quality".

University of California, Berkeley, online publication, "Selected Water Quality Indicators", CE112 Water Quality, Environmental Engineering.

University of Illinois Extension, online newsletter, "Mulching to Maim or Maintain Plants", Nancy Pataky, Home, Yard & Garden Newsletter, September 12, 2001.

University of Illinois Extension, "Watering Correctly Saves Time, Money, And Plants", Sharon A. Yiesla, Home Hort Hints Newsletter, Vol. 8, No. 4, August/September 2001.

University of Maryland Cooperative Extension, online newsletter, "Focus on pH and Lime", Nutrient Manager, vol. 3, number 2, Fall 1996.

University of Minnesota Extension, "Mulching the Home Landscape", Susan H. Barrott. July 1999.

University of Minnesota Extension, "Perennials: Covering For Winter", INFO-U #445, Doug Foulk and Deborah Brown, 1999.

University of Minnesota Extension, "Perennials - Uncovering in the Spring", INFO-U #444, Harvey Buchite and Doug Foulk, 2000.

University of Missouri-Columbia, "Water-Efficient Gardening and Landscaping", Denny Schrock, G06912. Reprinted September 1, 1998.

University of Rhode Island, "Are You Guilty of Over-Mulching?", Factsheets, Dr. Brian Maynard.

University of Rhode Island, "Irrigating the Home Garden", Factsheets, adapted from Diane Relf, Virginia Cooperative Extension, 2000.

University of Rhode Island, "Mulches", Factsheets, adapted from Donald H. Steinegger and Amy Greving, Nebraska Extension, 2000.

U.S. Water News Online, "Adirondack Lakes Displaying Signs of Lower Acidity", February 2002.

Utah State University Extension, "Take the Guesswork Out of Watering:, Garden Notes, Dennis Hinkamp.

Washington State University Cooperative Extension, "Watering Home Gardens and Landscape Plants", George J. Pinyuh and Ray R. Maleike, June 1996.

Weisenhorn, Julie, Manual for Hennepin County Sustainable Landscape Site, Hennepin County Environmental Services, December 2001.

  • © Regents of the University of Minnesota. All rights reserved.
  • The University of Minnesota is an equal opportunity educator and employer. Privacy