Inorganic Mulch

Solarizing Black Plastic Mulch

Inorganic mulch can be a very effective method of garden weed control.

This page details how to use mulch in the inorganic category.  

The gardener can determine which is best at preventing sunlight from germinating weed seeds, just the same as organic mulch types does.

1) Solarizing Black Plastic Mulch

This method is placing black plastic over an area where where weeds need to be eliminated. The plastic forms an impervious physical barrier that the sun can not penetrate.

This method takes about two weeks to kill weeds. The term solarized is used because the sun magnifies the heat underneath plastic raising temperatures aiding in killing garden weeds faster. If left on a very long time soil nutrients can be lost.

  • If flowers and plants are present water the area well before laying plastic. If watering could be difficult place a seep hose underneath plastic. Place seep hose underneath if you are planning on leaving plastic on longer than two weeks.
  • Place slit holes in plastic to have plants and flowers exposed. If plants and flowers get larger be sure to make holes bigger in plastic.
  • Because no air and limited water reach the soil it is not recommended to leave black plastic mulch on longer than two months. If you do leave on longer:

    • Place gravel or bark mulches make it more attractive.
    • Using gravel or bark mulches also helps with anchoring.
  • Be prepared to hoe the edges. Weeds always attempt to reach where there is sunlight.

At the end of gardening season make sure black plastic mulch is pulled up. This is not a winter mulch! Just as heat is magnified so is the cold. When frosting season comes do not leave this on. The cold will be magnified and cause plant and/or root damage, possibly killing plant.

2) Biodegradable Plastic Mulch

Use in the same manner as with the solarizing black plastic mulch.

Biodegradable plastic deteriorates over time. Because deterioration doesn't happen all at once, plastic breaks apart into strips which start to fly around on windy days.

  • Not considered as effective as solarizing black plastic method.

3) Newspaper

Look at this as a temporary solution. For the beginner gardener who may not have other mulch types available this is a great start. It also something that you probably already have, therefore saving you money.

This is also great for protecting anything that may have been planted or is sprouting in spring should a post winter frost hit randomly.

As an inorganic mulch:

  • This is a time consuming method of garden weed control.
  • Can take up to a season to be effective.
  • Use 6 layers of newspaper to avoid ripping. It is important to anchor down.
  • Newspaper will need to be replaced and re-applied as gardening season progresses.
  • Newspaper definitely falls under the category of inorganic mulch - it doesn't break down.
  • Other kinds of mulch can be placed over it to reduce the number of times newspaper must be re-applied.

4) Brown Paper Roll

As far as different paper methods this is the best option. Brown paper from the roll is a type of inorganic mulch that will rot and break down in time.

  • Pre-cut any slits necessary to expose plants and flowers in your garden.
  • Anchor down.

5) Weed Control Fabric

Also can be called Weed Fabric, Black Weed Fabric and Landscape Fabric.

Black weed control fabric is porous and breathable allowing water to get through. It does block the sun but this method of inorganic mulch is not nearly as quick killing weeds as the solarizing black plastic.

  • Slit areas where plants and flowers will be exposed.
  • Can be left on longer than a growing season because air and water can travel through.
  • Fabric does not break down. Fabric is also light weight and will blow on windy days. Use bark mulches and gravel to anchor down and help with being more appealing to look at.
  • If you have a great amount of cloth it can be wrapped around plants rather than placing slits. This way it can be used multiple times. Moisture can sit in between the folds allowing possible molding.

This is also a great temporary cover should there be a pre- or post-winter frost. Like newspaper, keep it in place one or two nights. It can be removed quickly and increase temperatures by 3 to 10 degrees, which can mean life or death to the plant.

6) Gravel

Gravel is good for drainage and is more decorative for landscaping.

  • Apply 2" thick if using gravel as a mulch.
  • There are two types of gravel:

    Washed Gravel, also called River Stone - Shapes are round to oblong because the water from the river has washed over the stones eroding them, there are no sharp edges.

    • Available in different colors and sizes.
    • Most common is Pea Stone.

    Man Made Gravel - This is crushed rock, which comes in various sizes. Gravel is the smallest size.

    • These can vary in product type and have sharp edges
    • Most common is Granite.

Bags of gravel can be purchased at garden centers and big box stores. If you won't be using a lot of gravel it is probably less expensive to purchase at one of these retail outlets.

If you will need a few tons consider local stone yards. It is less expensive to take this route but remember there will probably be a delivery fee on top of the price of gravel.

7) Rubber Mulch

This inorganic mulch is made from recycled tires (all steel is removed) and is available in nuggets and shredded styles. Most widely available colors are reddish-brown or black rubber mulch.

The Positives

  • Retains moisture very well because it doesn't absorb water like shredded bark mulch and others do.
  • Able to keep soil warmer by an additional 2 - 3 degrees over bark mulches.
  • Reduces bacteria and fungal growth.
  • Because rubber doesn't retain water, weed seeds are dehydrated lowering the percentage of new weeds that surface.
  • Rubber mulch is very durable and longer lasting. You should get about 4 seasons from rubberized mulch.
  • Great for traction. Very skid resistant.

The Negatives

  • Can be more expensive.
  • Can leach some chemicals into the ground.
  • If ignited, rubber mulch can smoulder and be hard to put out.
  • Water can't pass through as easily as other types of mulch. It can also float.
  • It doesn't break down as wood mulch does and can be difficult to plant around.

8) Glass Mulch

Glass mulch is made from recycled glass and comes in many different colors and sizes.

Like gravel, glass mulch is used for pathways and decorative landscaping purposes.

It is also used in fire pits, aquariums and counter tops.

Glass mulch can be very attractive and unique. It is used in the west and southwest regions of the country, but gaining in popularity in the east. Of all the mulches, this is the longest lasting inorganic mulch. Glass takes hundreds of years to break down.

  • Best used with sandy soils. It packs loosely and is good for drainage.
  • There is commercial and homemade glass mulch, but...

    Commercial glass mulch is rounded out by mixing with sand. It is processed more and there are no sharp edges.

    • No taste or odor.
    • It is small so there may be concern for pets ingesting it.

    Homemade glass mulch is also made by mixing with sand. Unfortunately there can be risk with homemade and wouldn't make a good option for inorganic mulch. Edges will likely be sharp, rather than rounded out well. Apply common sense here.

    • Not recommended for use in garden for fear of injury when weeding.
    • Not recommended for use on pathways, especially where kids and pets are present for fear of injury.

Prices for commercially made glass mulch can be moderate to expensive.

There are some cities across the country that have started to incorporate making glass mulch from the recycled glass they pick up. These prices can be very inexpensive.

Don't use a few things for inorganic mulch:

  • Be aware if using glossy papers, tar paper or particle board materials that they have poisons that can be released into soil effecting growth of vegetation around them.

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