Tag Archives: compost mulch

Covering the ground – our free mulching options

Oh look! I made a little display board.

Mulching is what enables us to maintain our garden to the standard we want, particularly keeping the new herbaceous plantings free of invasive weeds. Being economical gardeners, we don’t ever buy mulch in but rely on the resources we have here. I laid our main options out to photograph them.

Gravel mulch

Mulching with gravel at Wisley

I have seen gravel used at the RHS Wisley Gardens, particularly in the Piet Oudolf borders and the Missouri Meadow. In its favour, it is weed-free and it makes a good seed bed in that managed meadow where seeding down is encouraged. However, it is heavy to handle, expensive and, in a situation with herbaceous plants which need digging and dividing, it is inevitable that a fair amount of it will end up in the soil even if you make efforts to scrape it to the side. I am reluctant to use it and that is a pity because we have a small mountain range of it retrieved from the capillary beds when we dismantled them. Maybe 10 cubic metres of it and that is a lot.

Granulated pine bark is a stable mulch and a neutral dark brown. I doubt that it comes cheap if you are buying it for this purpose. Not only do we have a small mountain range of gravel, we also have what is referred to here as ‘the bark slug’. When we ran the nursery, everything was potted into granulated bark and Mark decreed that the bark was not to be re-used when plants were being re-potted. His position was that the bark potting mix was one of the cheaper inputs overall (he didn’t pay the bills; I was often somewhat shocked at how much the bark bill came to each month at around $750 a truck-load) and that using fresh bark cut down on weed contamination and disease issues. All the used bark went out to the bark heap and as the heap grew, it seemed to take on a life of its own and creep out like a sand slug.

Granulated pine bark – after at least a decade and fresh, but dry

We don’t re-use the old bark as a garden mulch any longer because of the fertiliser bubbles within it. We always used Nutricote as a fertiliser for commercial plants and very good it is too. But, and it is a huge but, the coating on the fertiliser granules remains long after the actual fertiliser has been used. Mark initially rejected the use of the waste bark on paths and gardens because his eye zoomed in immediately to the fertiliser bubble casings within it. Now we are even more concerned that it does not appear to be biodegradable and we don’t want to be spreading a non-biodegradable product through the environment.

This is not a problem that you will have if you buy in bark chip because it won’t have fertiliser added (and not all fertiliser comes with a coating). To the right, is fresh(ish) bark that we still use as a potting mix. To the left is what it looks like after more than a decade in our bark heap. Despite being an organic product, pine bark does not decompose readily.

Commercial chipping at the top, our home chipper below

Next is the woodchip mulch after nearly two years on the garden. It is not my favourite mulch but we were given a large truckload of freshly mulched copper beech branches and leaves and I needed to cover a few hundred square metres of newly planted garden. Beggars can’t be choosers. It was put through a commercial mulcher and is much coarser than we get out of our domestic mulcher machine. It is very light to handle and forms a crust across the surface, discouraging weed growth. I just don’t like the colour – it is pale creamy yellow when it goes on – and because it is so coarse, it takes a long time to mellow out to something more neutral. And I don’t like the coarseness. It looks… utility, which indeed it is.

Our home-generated woodchip, being of a finer texture, discolours far more quickly and is less dominant visually. But it takes a lot of prunings to generate much quantity.

Evidence of nitrogen robbing at the top – plants are sparser and showing a yellow tinge while others have thrived

The advice with woodchip is to be careful to lay it on the top because it robs the soil of nitrogen as it decomposes if it is incorporated into the soil. Behold an example. I did the initial planting of this aster and laid mulch. I must have gone back into the area to add some more plants. Clearly, I was not sufficiently careful to scrape the mulch to one side and some of it was dug in when planting. The somewhat bare area and yellowish tinge to the plants are signs of nitrogen deficiency. I keep telling myself to get out and scatter a bit of blood and bone on the affected plants to combat this but we do not generally add fertiliser to our garden so I haven’t got around to it yet.

Leaf litter mulch

Smaller leaves look tidier

Next up is leaf litter – natural, free, nutritious, enabling a healthy soil ecosystem but untidy for small, tightly maintained areas. The birds will scratch amongst it (which is a sign of a richer soil environment because they are looking for food) and it often needs raking back into place until it builds a good under-layer. I like leaf litter mulch visually but where I am using it is in larger, more naturalistic spaces. It would not be my first choice for small, confined areas. Though the smaller your leaves, the tidier they will appear.

Compost is king

And finally compost – our preferred mulch by a country mile. Because our soils are so friable, we generally add compost as a mulch rather than digging it in around the roots of the plants. The worms will do the work and incorporate it over time. We often choose to put the woodchip and leaf litter through the composting process first. Compost is light to handle, natural in appearance and makes a major contribution to soil and plant health. We make a lot of compost here, putting it through a process that heats up sufficiently to kill off many nasties but even so, we try and avoid putting seed heads and invasive plants on the compost heaps. The problem is that even though we have compost mounds that are turned by tractor, we don’t make enough to compost to meet our mulching needs which is why we sometimes have to go to alternative options.

Our gravel mountain (with a pretty apricot foxglove seeding into it)

Upon reflection, I may have to turn to the gravel mountain to mulch the new grass garden that I plan to plant this autumn. It is about 450 square metres and laying an 8cm mulch across that area is going to take a lot of whatever I use. At least this is a planting that I do not anticipate having to do frequent digging and dividing.

Outdoor Classroom: guide to garden mulches

There are five good reasons to mulch your garden:
a) Mulches suppress weeds when laid to a depth of around 6cm (but the mulch itself needs to be free of weeds).

b) Mulches stop soils from drying out as quickly by slowing evaporation. However they should not be laid on ground which is already dry because they will act as a barrier to stop water soaking in easily when it rains.

c) Some mulches will feed the soil and add valuable carbon content.

d) Mulches protect your soil from wind, torrential rain and erosion and may slow the leaching out of goodness.

e) Most mulches make a garden look much more attractive and reduce dust.

ORGANIC MULCHES

1) Pea straw is popular but when used in areas where peas are not grown commercially, it has a heavy carbon footprint in transporting it. It should be weed free. It adds carbon content to the soil but it is a myth that it fixes nitrogen (peas store the nitrogen in their roots but you are only buying the harvested tops of the plants). It is better in the edible garden, and great for strawberries, but not very aesthetic in the ornamental garden. A bale should cover about 6 sq metres. Water it well or the dry straw may blow away. Oat straw and barley straw are suitable substitutes.

2) Composted bark is widely used and lasts a surprisingly long time. Compounds in the bark stop it rotting down quickly. It adds carbon over time, it should be free of weeds and it is visually discreet in the garden because it is dark brown. Buy pre-composted bark if you can – it does not rob the soil of nitrogen as it breaks down. You can buy small bags but it is cheaper in bulk. A cubic metre should cover about 15 square metres.

3) Leaf litter is readily available and free in autumn. You can either disperse the litter over the garden beds immediately or you can rake it into piles and let it start to decompose first. A leaf rake makes this task much easier than trying to use a garden rake. Leaf litter will enrich the soil but it may look a little untidy in some garden situations.

4) Wood chip, sawdust, and wood shavings are not composted so must be laid on top of the soil and never dug in until they have rotted. This is to avoid them depleting the soil of nitrogen as they break down. Sawdust or shavings must be from untreated timber (tanalised timber is toxic) and fresh sawdust can look garishly orange on the garden. We use the fine wood chip from our mulcher extensively, raking it out immediately to a layer of about 6cm.

5) Old carpet must be natural fibre (generally wool), not synthetic. Test it with a flame – if it melts, it has synthetic fibres. Cutting with a Stanley knife is probably the easiest way to get manageable pieces. It may be best in the vegetable garden because it is not attractive. Lay it with the hessian backing side upwards and you can camouflage with leaf litter. Similarly, newspaper is not an attractive looking option, but it will work if you lay down a thick layer of maybe ten sheets at a time. Coloured pages are fine but avoid any glossy paper. It needs to be weighted down and covered with other garden litter to keep it moist. It represses weeds and adds carbon content.

6) Compost is our preferred option by a country mile but if you are using your own compost, you need to make sure it is as free of weed seeds as possible. Compost adds most of the nutrients the soil needs and is excellent laid as a mulch on top. Let the worms gradually incorporate it into the soil. It looks unobtrusive while nourishing and protecting the soils.

INORGANIC MULCHES

7) Stones are heavy to handle but can be visually effective in the right setting. They also store warmth for plants which prefer hotter conditions. They are best used with permanent plantings where the ground will not need to be cultivated (dug over) as you do not want to have to move the stones. Use a leaf blower to keep the build up of litter removed.

8) Limestone chip can be quite a stark white when first spread but this sometimes suits modern gardens. It is weedfree but it adds nothing to the soil. Once you have laid it on the garden, you will never get rid of it. Acid loving plants like rhododendrons and camellias will turn yellow because the alkaline lime will leach out. The same goes for crushed shell, which is also alkaline. The only way to keep these mulches clean and smart is to use a leaf blower to remove detritus.

9) Fine gravel and scoria are similar options. Fine gravel is a traditional mulch for rockeries and alpine plants because it is free draining and a neutral material. It can look a little industrial or utility in other settings. Scoria will give a decidedly retro 1970s’ look.

10) Weedmat is a commercial product designed predominantly for nursery use. It suppresses weeds and allows rain to penetrate while preventing excess evaporation. It should be laid taut and secured with wire hoops. It has no aesthetic value at all and looks uniformly unattractive when used in home gardens. It is at least better than its precursor, black polythene, which should never be used as a mulch because it sours the soil. Weedmat is bought by the roll and widths vary but it should price out around $1.25 a square metre.