Mulching in the month of May

Posted by John Stolzle, Jefferson County Extension

April Showers Bring May Flowers or, as some may call them, weeds.

Flowers such as Dandelion (Taraxacum officinale), Clover (Trifolium repens), and Henbit (Lamium amplexicaule), are important sources of energy for pollinators; Dandelions are especially important early in the spring. But their presence is not always wanted and managing these stubborn plants can be fraught with difficulty and challenging decisions. 

Mulch can be an excellent tool for managing weeds and can improve a garden’s overall productivity. A proper mulch can help to conserve and stabilize soil moisture, moderate soil temperature, control erosion, and in some cases promote soil microorganism (to the benefit of plants!).

A single mulch type does not fit all scenarios. Vegetable gardens, perennial flower beds, and trees can benefit from mulch, but specific types can be better for specific scenarios. This blog post offers a few tips and tricks for 3 common types of mulch: wood chips, black plastic, and grass clippings. In-depth information for mulching can be found in the links at the bottom of this post.

Perennial Flower Beds:
In a perennial flower or shrub bed, wood chips can help maintain soil moisture and reduce the need for watering. With wood chips, as with most mulch types, depth matters! Wood chips should be left on top of a soil. Wood which has been worked into a soil will eventually break down, but the process of decomposition can reduce a soil’s oxygen levels and tie-up soil nitrogen (to the potential detriment of plants)! While mulch can help to conserve water, watering regimens should be adjusted after laying down mulch, as prolonged periods of overly wet soil can lead to root rots! For wood chips of smaller sizes, 1-2 inches of depth should suffice for general weed control and water conservation; for larger chips of wood, 3-4 inches should do. 

Wood chips can make annual cultivation quite difficult (and can result in wood being mixed in with soil) and so generally, wood chips are not recommended for vegetable gardens or annual beds.

Vegetable Gardens:
Black Plastic
The use of black plastic, weed barriers, and fabric mulch is a challenging topic. Generally, these are NOT recommended for perennial beds, trees, or any areas with plants to be mulched longer than a single growing season. In a vegetable garden, during only a single season and mainly for warm-season crops, this type of mulch can really shine. Black plastic mulch can work very well with drip irrigation systems (beneath the mulch) in vegetable gardens, though care to not overwater should be taken. Black plastic can warm a soil, allow for earlier planting, and result in more rigorous early growth for certain plants (it is particularly great for tomatoes and other plants in the Solanaceae family). 

In the early season, a few extra degrees can really help plants get started; in mid-summer however, depending on a garden’s location, this warming factor can harm plants by overheating the soil. Some types of black plastic can also degrade in full sun. If plants are started early enough, they may grow large enough to shade the plastic and prevent this overheating; in other cases, other types of mulch can be placed on top of black plastic, though piling on too much additional mulch can reduce soil oxygen, so take care to not over do it! 

It is very important to the long-term health of a soil that black plastic or fabric mulch be picked back up after a growing season. Black plastic is not recommended as a permanent or even year-round mulch. Black plastic and weed barrier fabrics, even those with “pores for oxygen exchange”, can become clogged after a season of use, especially in clay soils. This leads to an environment where water and air are unable to penetrate a fabric. The problems just seem to mount when these types of mulch are used for more than a single season, without being picking up and removed between growing seasons. As an aside, these types of mulch can also girdle (“strangle”) trees as they grow, prevent air exchange (something tree roots need), and hamper a tree’s water supply.

All this is not to say these types of mulch are horrible choices. They can be extremely effective at controlling weeds and conserving water in vegetable gardens. Gardeners should just be aware that this mulch has fairly unique use requirements.

Grass Clippings
Grass clippings can be a great mulch for vegetable gardens. They can be mixed into a soil at the end of a season and will add nutrients to a soil as they breakdown. Grass clippings should be dried before application or they can become matted which can lead to a whole host of other problems. Recommendations for depth are around 1-2 inches. Multiple applications of grass clippings may be needed as the grass decomposes throughout a season.

One major consideration for grass clipping, especially for clippings obtained from third-parties, is whether herbicides might be present in the clippings. Herbicides are often applied to lawns, and the residual presence of an herbicide can greatly affect plants grown under herbicide-laced grass clippings.

A more thorough discussion on plastic mulch, wood chips, and a few other mulch types can be found in this factsheet on ‘Mulches for the Vegetable Garden’: http://cmg.colostate.edu/Gardennotes/715.pdf

Additional Resources:
CSU Extension has put together a wonderful overview for many different mulch types (including newspaper, crushed corncobs, straw, and pine needles) which can be found here: https://extension.colostate.edu/docs/pubs/garden/07214.pdf

More general information on mulch, specifically wood and rock mulch, can be found here: http://cmg.colostate.edu/Gardennotes/245.pdf

For Trees:
Wood mulch can work well, but care should be taken to avoid mulch volcanoes and mulch should be placed 6 inches away from the trunks of trees! Here is a great blog post on this very topic: http://csuhort.blogspot.com/2013/04/mulch-more-than-just-pretty-face.html

For further reading on Mulching Trees, here is a link to a document on this exact topic: http://cmg.colostate.edu/Gardennotes/658.pdf

Lastly, in case it may be of interest, here is a great blog post on garden irrigation systems: http://csuhort.blogspot.com/2019/04/guided-tour-of-basic-drip-system.html

May your May be filled with gardening success!