How to Choose the Right Mulch for your Garden

Chinese cabbage 'Tokyo Bekana' growing in chunky palm frond mulch.

Chinese cabbage 'Tokyo Bekana' growing in chunky palm frond mulch.

Mulch can offer many benefits for your garden, but if you don’t know how to choose the right type for your needs, you might be doing more harm than good.

Types of Mulch

Generally, there are two types of mulch. The type you choose depends on your needs. If you want to cover the soil to retain moisture, limit water usage, or suppress weeds, you need ground cover mulch applied on top of the soil. If you want to improve and condition your soil, you need soil conditioner mulch applied on top of the soil or gently mixed into the first couple inches/centimetres of soil.

Ground Cover Mulch Benefits & Characteristics

Ideal ground cover mulch: Chunky hardwood chip.

Ideal ground cover mulch: Chunky hardwood chip.

Ground cover mulch can save water by keeping the ground moist, suppress weeds, and slowly release nutrients into the soil. Only chunky mulch with 5% or fewer fines, 0% fines is ideal, offers these benefits. Fines are small particles that may have a soil like texture. Using mulch with greater than 5% fines will not deliver the benefits expected of ground cover mulch; in fact, it may have the opposite effect. Chunks of wood or bark make a good ground cover mulch, but wood is best.  Pieces should be ½” (15mm) to 1 ¼” (30mm) in size. Apply on the soil surface at 2” to 4” (50-100mm) deep. Hardwood chip meeting these specifications is good long-lasting ground cover mulch.

The Problem with Some Ground Cover Mulches

Dwarf fruit trees and perennial herbs growing in chunky hardwood chip mulch.

Dwarf fruit trees and perennial herbs growing in chunky hardwood chip mulch.

The key to good ground cover mulch is airflow. If there are too many fines, airflow will be poor and the soil surface will remain wet for too long. Poor airflow combined with constantly wet soil can contribute to crown or root rot and disease. Humid climates compound these issues.

Mulch with fines can also bind together to make a mat that repels water, having the opposite effect intended. Unless a considerable amount of water is applied, it can’t penetrate the mulch to reach the soil surface. Chunky mulch with little to no fines allows water to reach the soil but also keeps the soil surface cool, thereby reducing evaporation and saving water.

Ground cover mulch that contains fines creates the ideal environment for weeds. While it may suppress weeds under the mulch, the fines trap seeds that blow onto the mulch, which act like a soil, allowing them to germinate and take hold due to excess moisture. Seeds that blow onto chunky mulch either fall down to the soil where their growth is stifled by the mulch, or they germinate on the mulch surface and the spaces between the mulch prevent the roots from taking hold before dying.

Lastly, uncomposted mulch containing many fines causes a condition known as nitrogen draw down, making nitrogen unavailable to plants until the composting process is finished. Composted mulch containing 5% or greater fines won’t tie up soil nitrogen, but still has the other problems mentioned. Chunky mulch takes longer to degrade; releasing nutrients slowly over time and avoiding nitrogen draw down.

Soil Conditioning Mulch Benefits & Characteristics

Soil conditioning mulch improves sandy soils by adding organic matter, increasing water-holding capacity. It breaks up clay soils, improving drainage. This helps reduce watering requirements, retains fertiliser in the top layer of soil where plants need it most, and slowly releases nutrients. It also adds beneficial fungi and bacteria to the soil, boosting plant health. Thoroughly composted fine grade mulch is ideal for soil conditioning. Apply at a depth of 2” (50mm) on the soil surface and allow the earthworms to "till" it into the soil for you, or gently incorporate into the top 2" (50mm) of soil.