How Much Wood Chip Should I Apply and When?

The amenity wood chip supplied by Gristwood and Toms is typically aged for about 3 months, which gives it a rich and attractive brown colouring. We recommend beginning mulch application before annual weeds become established. You should ideally mulch bare soil as soon as practical, especially in the spring and autumn when weeds are most likely to germinate. While you can apply mulch at any time, be careful in Spring not to heap mulch on top of emerging and growing plants. Autumn is ideal after herbaceous plants have finished, when the soil is damp but still relatively warm.

For the best results, wood chip mulch should be deep enough to suppress weeds and inhibit light to the soil surface: research has demonstrated that weed control is directly linked to mulch depth, as is enhanced plant performance. In fact, shallow mulch layers may promote weed growth and/or require additional weed control measures. We recommend at least 75mm depth for ornamental sites.

Remember, do not to pile mulch against the trunks of shrubs and trees. This creates a dark, moist, low oxygen environment. Piling mulch on the trunk provides exactly the right conditions for fungi to enter the plant. Taper the mulch as you approach the trunk and this doughnut-shaped application will protect the soil environment as well as plant tissues above ground.

Once the mulch is applied, little management needs to be done other than reapplication to maintain minimum depth. Churn up the existing mulch first before adding a renewal layer. High traffic areas are most likely to need replacement.

The Facts – Are There Any Drawbacks With Using Wood Chip?

In this article, we tackle some of the commonly held concerns we’ve heard regarding the use of arborists wood chip as a landscape mulch material. It seems most of the concerns are contrary to the results of scientific research trials, so we feel it incumbent upon us to explain some of these ‘myths’.

Wood chip will ‘rob’ my soil of nitrogen and is bad for plants
In general terms, woodchip mulches do have a high carbon to nitrogen ratio and will increase the essential nutrient levels in soils and plant foliage. Fresh woodchip mulch (less than eight weeks old) can temporarily reduce the supply of nitrogen take up at the mulch/soil interface level, but this is a good thing as it will serve to inhibit weed seed germination. Established plant roots below the surface will not be affected and bulbs, tubers and corms are likely to thrive. It is, however, inadvisable to use high carbon : nitrogen woodchip in annual beds with fine, shallow root systems or on vegetable plots. If you still have concerns, the woodchip that Gristwood and Toms supply, has been aged for around 8-12 weeks. Further, as long as the wood chip has been on the site for at least six months, the C:N levels won’t create nitrogen deficiency in plants or crops if worked into the soil.

Wood chip will acidify my soil
There is no evidence to support this. It is very difficult to significantly change the pH level of the underlying soil without the use of chemicals.

What about diseases – can using chip from diseased trees infect my trees or plants?
Studies show that wood chip used correctly as a top-dress mulch material does not transmit pathogens several inches from the mulch, through the soil to the roots of healthy trees and plants. However, it is important to clarify that you should not use wood mulch as a backfill material or incorporate woodchip into the soil. Neither should mulch be piled up against the trunks of trees and shrubs as opportunistic pathogens can be transmitted this way.

Fungal species are generally decomposers, not pathogens. If you have good healthy soil, beneficial fungi will out-compete opportunistic pathogens (such as Phytophthora) anyway.

Will wood chips will attract pests?
No. In fact, wood chips act as a repellant to many types of pest.

Credit to Dr. Linda Chalker-Scott and her extensive research and scientific publications that have been used to inform and support this blog – in particular her ‘Horticultural Myths’ column.

Should I Buy Wood Chip or Bark Chip?

So, which product is best for your landscaping scheme? Bark chips or wood chips? Well, surely bark chips are the superior product because they are more expensive, right?

Well, no actually. While it is true that bark is certainly more expensive, research trials show that woodchip consistently outperforms bark regarding moisture retention, weed suppression, sustainability and temperature moderation to name a few – all the critical stuff that a good mulch should do.

So, let’s look at each of these factors in turn.

1. Wood chip retains and releases moisture better than bark
The bark is, of course, the outer covering of a tree. It is naturally waterproof and is designed to repel water. It does this because of a waxy coating called Suberin. This explains why fresh bark chips always appear to look dry. Now, a good layer of fresh bark chip on your planted area can create a near-impenetrable barrier that prevents water from reaching the plant roots – this is not good news for plants. Wood chips, on the other hand, are not coated in this wax-like substance and therefore can absorb, retain and release moisture. The woodchips will grab moisture from the air during the night, releasing it into the earth during the day. They will also reduce water evaporation from the soil when it is warm. Moisture is, of course, critical for plant survival and vitality.

Wood chip 1 – Bark Chip 0

2. Wood chip provide excellent weed control
If you want to reduce the need to spend hours and hours hoeing off weeds, and you support a sustainable, herbicide free environment, then natural mulch is the weed-control method of choice. Unlike the uniform nature of bark, wood chip is made up of bark, wood and leaves of varying size and density. This diversity of ingredients means that woodchip is more efficient at light reduction (preventing weed seed germination and the photosynthetic ability of buried leaves), allelopathy (inhibiting seed germination) and reduced nitrogen levels at the mulch/soil interface (reducing seedling survival). It also means that the product resists compaction, and it decomposes at different rates, gradually releasing beneficial nutrients into the soil (resulting in a more diverse colony of soil organisms which improves plant productivity).

Wood chip 2 – Bark Chip 0

3. Sustainability
Consider where the product you buy comes from and the complex logistics involved in transporting it to your location. Some bark products are imported from forests abroad. Some products are transported between multiple locations; from logging stations to processing plants, held in storage, on to main distributors and then the retailers before finally getting to the end user. All this adds up to many miles travelled. Whereas arborist wood chip can be sourced locally to where you live or work and is made from locally grown trees – trees that have been sustainably managed by local Councils and landowners. Our wood chip comes from essential tree works, is processed into various grades of chip and delivered straight to you. Fewer miles travelled, better for the environment and better for your pocket too!

Wood chip 3 – Bark Chip 0

4. Temperature Moderation
Mulch acts as an effective sunscreen to block the sun and moderate soil temperature, and it seems woodchip appears to be most effective at this job. In winter months soil covered in wood chip will stay warmer longer and may offer protection to plants from sudden changes in temperature, or where plants have not gone into dormancy early enough before the chill sets in. In springtime, the soil will warm faster, helping to extend the plants’ growing season.

Wood chip 4 – Bark Chip 0

So, the question remains, why do some people insist on using bark? It seems that there are two reasons.

1. It is what they have always bought.
2. Aesthetics – bark looks pretty.

Let’s exclude number one straight away because that is not a good reason to do anything, is it?

So we are left with the aesthetic argument – that bark looks nice, it is visually attractive and consistent in size, colour and texture. Okay, maybe – but surely, this must be secondary to the primary function of the product and the health of the plants it is being applied to enhance. As we demonstrated above, not only is wood chip hands-down more effective as a landscape mulch, but bark can be detrimental to the health of plants. Moreover, if you are still concerned about looks, we can produce a wood chip mulch to a consistent uniform grade, or nugget style chip that looks very attractive and will enhance any landscape scheme.

References:

https://puyallup.wsu.edu/lcs/