Gardening Topic for November 2006
The What, Why, When and How of Mulching

Provided by the Western Massachusetts Master Gardener Association



By Julie Abramson, Master Gardener 



Mulching garden beds can often make the difference between success and failure, especially when gardening on dry sites or with newly planted perennials, shrubs or trees. Some plants that are marginal for your climate zone may also thrive in your garden with winter protection through mulching. Mulching is one of the simplest and most beneficial of gardening practices.

What is mulch?

It is a protective layer of material, often organic in nature:
• Grass clippings
• Straw
• Bark chips
• Newspaper
• Cardboard
• Shredded bark
• Compost
• Leaf mold or decomposing leaves
• Cocoa or buckwheat hulls
• Shredded leaves
• Pine needles
• Christmas tree boughs

Such material is laid down on exposed soil in the garden and spread to a depth of 2-4 inches. Mulch can also be inorganic, including stones, stone dust, landscape fabric and plastic sheeting.

Why mulch?

Organic mulches:

• enrich and protect the soil from erosion
• reduce compaction from the impact of heavy rains
• conserve moisture
• prevent weed growth
• keep soil cooler in summer and frozen in winter (see ‘when to mulch’ below for explanation of why it is important to keep the soil frozen in the winter)
• keep fruits and vegetables clean and keep soil-borne disease from splashing onto plants
• set plants off visually and give garden a finished look
• improve the condition of the soil by providing organic matter from decomposing mulch
• improve root growth and the infiltration and water-holding capacity of the soil
• provide source of plant nutrients and encourage earthworms and other beneficial soil organisms

Inorganic mulches can do some of the same things that organic mulches do, such as keeping weeds down and protecting soil and plants; however, they do not enhance the soil, provide nutrients to plants or improve the look of the garden, with the exception of stone mulches.. Stone mulches can be very effective in rock gardens both visually and as a protection from rot for sedum, cacti and other rock garden plants. Plastic mulches offer some advantages for the vegetable garden as they warm the soil and keep warmth-loving plants such as tomatoes, peppers and eggplants happier in our northern climate. Recent research has shown that red plastic mulch improves the productivity of tomatoes. Salt marsh hay and straw work well in the vegetable garden while other types of hay may harbor weed seeds. Shredded leaves are an excellent mulch for perennial beds, as is shredded bark; while bark chips and shredded bark are effective in foundation plantings or around bushes or trees.

When to mulch?

Time of application varies with your goal in mulching. Mulches provide an insulating barrier between the soil and the air and moderate the soil temperature. Therefore, a mulched soil in the winter may not freeze as deeply as one that is not mulched. Yet, mulch also will cause soil to stay frozen when warm spells occur in the winter, thus protecting your plant roots from exposure due to the heaving resulting from freeze and thaw cycles. This is a major cause of plant loss in the garden. Although it may seem counter-intuitive, when you mulch to protect plants from the impact of winter, you are actually trying to keep them consistently cold rather than warming them with the mulch. Therefore, it is essential to wait to mulch plants for the winter until the ground is fully frozen. In our area, that may vary with the weather but often coincides with the post-Christmas disposal of Christmas trees whose boughs serve as an excellent mulch for the perennial garden. In my last garden, I was able to convince the town waste collectors to drop a load of Christmas trees at my door when they were collecting them for recycling. In addition, if you apply mulch before the ground has frozen, you may be offering rodents an attractive spot in which to over-winter and possibly munch on the bark of trees or shrubs. Mulching around trees also can protect them from weed whackers and mowers.

However, since mulch insulates, mulched soils warm up more slowly in the spring than soils that are not mulched. Therefore, you should not apply mulch in the vegetable garden until the soil has warmed. In fact, you can use plastic mulch to hasten the warming of the soil for those vegetables that appreciate warmth but not for those such as peas or lettuce that need cooler soils. Wait to add mulch to existing perennial beds until the soil has warmed fully.

How to Mulch?

• Remove weeds before spreading mulch.
• Keep mulch at least 6 inches away from the trunk or a tree of shrub to avoid build-up of moisture and provision of cover for rodents
• Do not let mulch come in contact with the crowns of plants
• Add no more than 2-4 inches of mulch
• Avoid mulch build-up beyond 2-4 inches especially around trees; build-up can reduce oxygen available to plants and trees
• Put down several layers of newspaper or a layer of cardboard when starting new beds; this suppresses weeds initially and adds organic matter to the soil as it rots
• Purchase mulch by the yard if you have a large area to mulch or by the bag at garden centers if your area is small

Supplementary source: The Natural Resources Conservation Service 

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Provided by the Western Massachusetts Master Gardener Association