Anyone involved in horticulture in London will have observed how dry the soil has been in recent months. April was the warmest on record for the UK with a distinct lack of April showers. And really, we’ve hardly had any rain since (Lewis wrote this blog before he headed off on holiday to sunny climes * so he’s not had the pleasure of enjoying the torrential downpours that London has been experiencing recently!). 2015 is on track to be the warmest on record, beating 2014 and continuing the dangerous trend of warming that is currently taking us into unchartered climatic territory.
This is not great news for anyone, not least for newly planted fruit trees; the first year is a crucial one for good root establishment. A tree is ‘established’ once it has grown enough roots to keep it alive without the need for supplemental irrigation and growth rates become more or less consistent from year to year. The faster the roots grow, the sooner the tree becomes established. However, new roots will only grow well in moist soil. So, as good orchardists you’ll certainly be watering weekly if it’s your orchards’ first or second year. There’s a simple thing you can do to both maximise the impact of your watering effort and your tree health…
Mulch has become something of a mantra for us at The Orchard Project. That’s because it is such a simple action that yields multiple benefits. Once you’ve read this, you’ll understand why we bang on about it so much. Simply defined, mulch is a layer of bulky organic (once living) material placed around the base of a plant, primarily to conserve soil moisture and prevent competition from unwanted plants. But it really is so much more…
Conserving Moisture and Water Retention
A lack of sufficient water can be a key limiting factor to good tree growth. Fruit trees will only expand their roots sufficiently when there is enough water in the soil. A drought stressed tree may become stunted and is more susceptible to attacks from pests and disease; aphids and powdery mildew, two frequently occurring fruit tree enemies, can both occur more readily in a stressed tree. Watch this video for more details on that.
A layer of mulch acts like a ‘lid’ that allows water into the soil but helps to minimise it’s evaporation, meaning that your weekly watering goes much further.
When you water, be sure to water your whole mulch circle so that there is plenty of moist soil for the newly formed roots to spread out into.
Mulch also adds organic matter to the soil which breaks down to form humus. Humus can be thought of as the ‘life-force’ of the soil. Essentially, it is the end product of the break-down of organic matter and it provides the ideal habitat and food source for many soil microorganisms. It can hold 80-90% of its weight in moisture, meaning that a humus-rich soil is one that is more drought tolerant.
Preventing Competition
Another key role of mulching is to prevent sunlight from reaching unwanted plants around the base of the stem, thereby preventing them from growing too close to your tree where they will compete for nutrients and water. Grass is very competitive in this sense with its dense mat of roots and is allelopathic meaning that it produces biochemicals to inhibit the growth of the tree (as ultimately the grass doesn’t want the canopy to shade it out). Plant two of the same tree into some turf grass, mulching one but not the other and after a few months of growth you’ll see the obvious difference; the unmulched tree will be stunted compared to the mulched one.
A Visual Barrier
Newly planted trees in public spaces often succumb to strimmer damage, aka ‘strimmer blight’, where grounds maintenance workers get too close to the young stem and strip lashes of the bark unintentionally while trying to clear the grass and ‘weeds’. Plastic rabbit guards are no match for the spinning prongs of doom, being shredded in seconds. A healthy tree may well be able to take the odd lashing, being able to heal the wound by creating a callous. But if this happens repeatedly and a section of bark is completely removed, the tree will slowly die above that point, as without its phloem it will be unable to transport vital sugars from the leaves to the roots.
An attractive circle of mulch creates a ‘no-go’ zone as there should be no need to strim anywhere near the stem if your mulch is doing its job and supressing plant growth.
Woody Mulch for Woodland Species; Creating Fungal ‘Duff’
Most of the fruit trees we plant are descendants of the wild fruit trees of the Tien Shen forest; forest edge species. The soil in a forest is one dominated by fungi as fungi are specialists in breaking down the lignin that gives wood its hardness. The ratio of fungi to bacteria increases as more woody material is added to soil as biomass. In a woodland soil this can be 100:1, in contrast, where the dominant vegetation is leafy, like turf, the bacteria dominate as they can break down the cellulose more easily. We can shift this ratio by choosing a mulch type that is more fungi-friendly.
This means that if you continue to add rough woody material to the area around each tree you’ll be creating a fungal ‘duff’ zone around each tree, thereby creating ‘islands’ of optimum soil for your fruit trees.
Wood chip is an ideal mulch for fruit trees. It is long lasting, develops a lovely humus, attracts a fungal frenzy and is available in large quantities for free in towns and cities, meaning we’re turning a waste product into a valuable resource. By adding it to the soil we are mimicking nature’s way of building forest soil from the top down.
Nitrogen Robbers?
Many people worry about using woodchip due to perceived notions of ‘nitrogen robbing’; a process whereby the soil microbes involved in breaking down the wood use up the available nitrogen in the soil, temporarily ‘locking’ it up. It seems, however, that this phenomenon is overstated. Here we’re talking about adding a layer of woodchip onto the soil surface, not tilling it into the soil. There may be a small loss of nitrogen at the interface between the mulch and the soil surface, but for a deeper root system this shouldn’t be an issue. In fact, this may even increase the mulch’s weed suppressant capability as it inhibits the growth of any unwanted seeds present. Finally, as this chip will sit on top of a mulch mat of some kind, either hemp or cardboard, it has a further 6 months or so to break down before even making contact with the soil.
Feeding the Soil
Mulch also adds nutrition, feeding the soil and then ultimately the host tree. By using thinner diameter ‘twiggy’ wood you are adding a higher ratio of cambium layer and bud tissue, both of which contain Nitrogen, Phosphorus, Potassium, Calcium and Magnesium, amongst others. It also provides an easy-to-break-down meal for the fungi as it hasn’t as much tough lignin as older wood, fungal ‘fast’ food if you like. This twiggier wood is known as ‘Ramial’ chipped wood (RCW) and is the ideal fruit tree mulch and replicates the natural twig and leaf fall for the forest.
Anything else?
Yes. Mulch will help to stabilize the soil temperature, keeping it cool enough for all of those vital soil organisms to keep up their work during the summer heat. It will help to prevent the loss of top soil during heavy showers. And it will provide habitat to a range of invertebrates in your duff zone.
Chip Species
Once you’ve contacted your local tree surgeon, it’s worth specifying that you want chip from hardwood species where there has been no signs of honey fungus. In the ‘Holistic Orchard’, Michael Phillips points out that softwood species don’t make a good mulch for fruit trees due to the way the wood decomposes:
“The ‘Brown rots’ transform softwood cellulose to produce polyphenols and allelopathic compounds specifically relied upon by evergreen species to suppress other plant species”.
Hardwood ‘White rots’ on the other hand support a deciduous environment which is better for fruit trees. Saying this, our friends at the Brighton Permaculture Trust have been using conifer chip for years and have not noticed any difference in tree health, so experiment with the resources you have access to.
White fungal mycelium spreading through recently added wood chip
Each tree should have a mulch ring measuring at least 1m in diameter, this can be expanded by 30cm each year in the first few years after planting to pave the way for new feeder root growth. Mulch should be a minimum depth of 5cm, and a maximum of 10cm. If this is the first time you’re mulching the trees be sure to add a mat layer first, either cardboard or a hemp mulch mat (if your trees were planted with The Urban Orchard Project they will have been mulched at the time of planting and the mulch mat should have killed off the grass beneath).
It’s important to make sure the ground is moist before adding the mat or mulch directly, so give the ground a good soaking if it’s dry. Make sure that the mulch isn’t touching the stem of the tree as the microbes may begin to work away at the damp bark; a ‘doughnut’ shape space between stem and mulch is what you’re after. Depending on who and how the grass ‘sward’ is managed around your trees you may need to communicate that you’ll be mulching outside of the tree cage and that this chip should not be removed or mown. Developing good relationships with the contractors involved can be beneficial here.
So, go forth and mulch!
* Lewis felt it important that the reader was informed that his holiday was to Europe in a vehicle powered by recycled vegetable oil to minimize the impact of the problem he was bemoaning!
Mulch your trees and they’ll thank you…