Mulches can stop weeds from growing in the garden, or at least slow them down, making mulches a great addition to an organic garden. They work by blocking sunlight. No chemical weed-killers are required! The choice of mulch can depend as much on the plants in the garden as on which mulches are available.
Kinds of mulches for organic gardens
A mulch is a material that we spread over the soil and around plants. Many materials are used for mulch, even plastic sheeting, but organic gardeners may want to use materials that will improve the soil as they decompose.
Examples of these preferred materials include old leaves (saved from the fall), pine straw, pine bark nuggets, and bagged soil conditioners (which are ground up “forest products”).
Pros and Cons of Different Mulch Materials
Old Leaves have the benefit of being free. They might need to be stored over the winter, if they are needed for the spring or summer garden. Also, they are not super-long-lasting. However, “free” can override those minor problems. They do pack down over time, so pile them on a little higher than the depth you are aiming for. Do not use leaves from black walnut trees — they contain a compound that is toxic to some plants. Leaves dropped from other trees should be fine.
Pine Straw is long-lasting, but it packs down over time, and it does seem to allow more weeds through than woodier mulches. However, near a house, pine straw has the advantage of not attracting termites. I use pine straw in shrub beds near the house.
Pine Bark Nuggets are long lasting. This makes them good for garden beds that will not need replanting any time soon. I use mini-sized nuggets, but others may prefer bigger pieces of pine bark. Over time, these will decompose and the mulch layer become thinner. Then, the bed will need to be re-mulched. If the bed needs to be reworked sooner, for any reason, the bark nuggets can be raked and removed, then re-used when the gardener has finished dividing/replanting or doing whatever task needed to be done.
Bagged Soil Conditioners are good to use in parts of the garden that need more frequent replanting. Soil conditioners are ground-up forest products (wood and bark), and they are highly variable between brands and even within brands, from year to year. This variability is caused by changes in where the pieces of tree are coming from. Regardless of the origin, the smaller size of the particles allows faster decomposition. This stuff doesn’t have to be raked away when reworking the garden; it can be turned into the soil, where it will help to improve the text and water-management of our Southern clay and/or sand soils.
Will it help to start with a layer of cardboard or newspaper?
My own experience is that I can use a thinner layer of mulch, and still get fewer weeds, if I spread a thin layer of paper or cardboard on the soil first. Since mulch can become expensive, this is a big help for the garden budget.
However, researchers at Washington State University published a summary paper, that you can read as a pdf, that includes six reasons why using newspapers or cardboard is not always a great plan.
Here is a summary of the reasons they give:
- The paper layers can become home to pests, including termites, which are attracted to cardboard.
- Mini nuggets and other wood-based mulches, when used properly, do a fine job of stopping weeds on their own.
- Wind can blow the paper and cardboard layers around.
- If the paper dries out, it can repel water, stopping rain and irrigation water from getting through to the roots of your plants.
- If the paper stays soggy, and it is on heavy clay soil, it may stop oxygen from getting through to the plant roots.
- Paper and cardboard mulches are not attractive.
The most compelling reason to me, in the list above, is the one about termites, but the document seems to refer to the use of very thick layers of cardboard and/or paper that are not covered by another mulch material.
Most gardeners are not using multiple layers of cardboard around their plants, or super-thick layers of old paper. We also are not leaving it uncovered to blow around, mostly because of that last reason: it’s not attractive.
In the past, I have laid cardboard in a single layer under pine straw in the shrub beds near the house. The information in the Washington State document about termites’ being attracted to cardboard has changed that practice. Now, when I re-do mulch under those shrubs, I spread a thin layer of paper instead of the cardboard.
How to mulch the vegetable garden
If you decide to use paper under your mulch in the vegetable garden, these are the steps to follow:
- Make sure that either all the plants are up and visible, or that the spaces where seeds are still below-ground are marked, so you don’t accidentally cover them up.
- Remove any existing weeds.
- Spread a thin layer of compost or a little fertilizer over the bed, to help the soil microbes whose job is to break down your mulch layers.
- Water the soil, since mulch can slow the rate water soaks through.
- Lay newspaper between plants, in layers just two-four pages thick; it shouldn’t touch the plant stems. Don’t cover places where you have planted seeds that are still under ground.
- Water the paper. This holds it down so it won’t blow away while you’re working, but it also improves contact with the soil. Water will get through the paper faster if it starts out wet.
- Spread your chosen mulch over the top. Keep it an inch or two away from plant stems.
Some weeds are tougher than others
Weeds that have large, long-lasting roots, like dandelions and the remains of shrubby plants, need to be dug out of the garden before any mulch is laid. They can rise up through a surprising amount of mulch.
Bermuda grass, which can be great as a Southern lawn, will come up through almost any mulch. I have seen it come up through contractor-grade weed barriers, through cardboard, through gravel. I have seen it grow up through Very Deep raised beds. It should be dug out of the garden before any mulch is laid.
The good news
Using mulch in the vegetable garden really can cut back on the task of weeding. Some weeds will probably come up around the edges of the garden and in any gaps that develop in the barrier created by the mulch, but the work of keeping weeds down will be greatly reduced.
Mulch also reduces water losses from the soil, which means the gardener can spend less time watering the garden. The past two summers, my garden has been fine with zero watering in the months of June, and July. Those were both generally “wet” summers here in the Atlanta area, but there were some dry stretches mixed in.
Another benefit of a mulch that decomposes fairly quickly, like soil conditioner or old leaves, is that they improve soil texture as they break down. As those mulch materials decompose, they provide some of the same benefits as compost. To read more about the use of compost in building good garden soil, see my blog post about the topic.