When to Mulch
The right time to mulch a garden is dependent on what type of plant material you are mulching and the weather conditions. When the soil is warming up from the freezing temperatures it experienced all winter in mid- to late spring is mulching season. Doing it too early will slow down the warming process, which the soil needs to do its job. Mulching too late in the fall can insulate the ground and prevent plant dormancy – a much-needed hibernation that helps plants survive the cold winter months.
How Much to Mulch
A layer of mulch on a planting bed helps suppress weeds, retain moisture, and moderate soil temperature. But the biggest problem we see today is too much mulch. When added year after year, mulch can actually harm plants by creating a layer that doesn’t decompose and doesn’t allow root growth. The soil becomes so matted that nutrients and water can’t penetrate to the roots, and the plants will suffer.
It is a good idea to strip off as much of the old mulch as you can and top dress with an inch of new mulch. If you start with stripped beds and add one inch a year, you’ll only have to strip them every three years. The goal is to keep the mulch layer less than three inches deep. Apply some low-nitrogen balanced fertilizer to the stripped beds before you put down the mulch. Make sure the mulch doesn’t touch the trunk of trees or shrubs.
The Worst Thing to Do with Mulch
Piling mulch up around trees is a very bad idea. Excessive mulch causes rot in the trunk, creates a home for insects that attack the tree, and encourages the development of a secondary root system. A tree with a secondary root system in the mulch zone starts to depend on it, causing the primary, deeper, root system to wither. That makes the tree vulnerable in a drought, when the soil is dry near the surface and the primary roots are no longer able to draw deep water. If the primary root system dies, the tree loses its anchor and is at risk of toppling over in heavy wind or snow.
Comparing organic mulch and inorganic mulch
The biggest difference between the two is that organic mulch breaks down into the soil over time, adding in nutrients. But inorganic mulch doesn’t dissolve, so soil quality pretty much stays the same.
Organic mulches (such as bark, grass clippings, wood chips or leaves):
- Gradually convert into plant soil adding nutrients to help plants grow.
- Help conserve soil moisture and even out soil temperature.
- Can be used for annual or perennial gardens, trees, shrubs, or fruit and vegetable gardens.
- Needs to be replaced at least once a year.
- Reduces evaporation which conserves water.
Inorganic mulches (such as stones, gravel or landscape fabric):
- Used for aesthetics, since they don’t help boost soil health.
- Are fine for permanent plants like trees and shrubs but aren’t a good option for perennial and annual gardens.
- Are good at deterring weed growth compared to organic mulch.
- More expensive up-front.
- Can throw off soil pH as small particles erode from rock.
- Can raise soil temps in summer.
More to Consider
Dark-color mulches will absorb and retain more heat from the sun than light-color ones. This is an advantage in cooler regions but a disadvantage in hotter climates.
Light-color mulches (particularly decorative landscaping types, such as white stones) reflect light and heat and can dangerously overheat surrounding plants.
Some mulches won’t stay put. Gravel and stones creep onto lawns (and make tempting throwables for kids). Small bark chips can wash downstream in a heavy rain. In general, mulches with heavy or large pieces are more likely to stay put.
Organic mulches, such as ground bark, manure, and compost, improve the soil. Stones and plastic don’t. Black plastic, unless it’s porous or perforated, grows a smelly, slimy coating. It also turns brittle and breaks into little pieces that escape the garden. Cheap landscape fabric is not worth it — weeds and roots will tangle in it.
The best mulch is the one that meets the unique needs of YOUR LANDSCAPE. Take stock of your plants to figure out what you need. If you’re relying on looks alone to cover your garden beds, a colored or inorganic mulch might be right for you. If growing flowers and fruits is your main goal, you should reach for organic mulch.