I am interested to hear people's thoughts about what mulch is best. Whilst I imagine the answer will generally be "depends on the circumstances", I am particularly interested to know where gravel mulch might be best and where it would be bad. I have had several conversations with fellow designers in Melbourne who would NEVER consider gravel for mulch as they believe it will transmit excessive heat to the soil and increase evaporation.
I have tried gravel mulch myself years ago and foolishly used a 14-20mm size. I think a pea gravel - say 7mm - might be preferable. The larger gravel drives me mad to this day - it is so difficult to garden in - can't easily dig plants out or dig new holes to pop new plants.
A further question is which organic mulch is best?
I am most interested in people's thoughts and experiences around this topic.
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Gravel garden is only good for a Mexican garden with cacti and agave. I know that in Europe it is used extensively for driveways, pathways and such but in my opinion it is too hot here to use it. As for organic mulch for vegetable garden I would use sugarcane or Lucerne mulch, for ornamental beds chunky wood mulch, NOT the fine black, composted mulch as it forms impenetrable layer after a while and you cannot take advantage of an occasional summer shower as water just runs of it.
I wrote a whole chapter on gravel mulch in The Deckchair Gardener (which may not be available to you?) based on Derry Watkins useful experimental work in gravel gardens.
Organic mulch – here in the UK I have had good results with fresh wood chippings and bark, in spite of the ‘experts’ ideas about that. But now I mostly mulch with the borders own contents – by cutting down and leaving at the end of the season: see https://veddw.com/general/just-cut-it-down/.
It is available here, Anne. I’ve not seen it in bookshops, but there’s always The Book Depository. Really keen to hear the results of any experimentation. It’d be so interesting to work out how rainfall (Wales vs New South Wales) changes the thinking, if at all
I find the sugar cane mulch a bit fine and tend the use it it in the chook shed then compost it. I usually use pea straw vegie on beds . My local council recycles our green waste and I find the course grade has an excellent balance between porosity and weed ssuppression. It is made by a company called Twigg. It has pieces from5 to 6cm long and three wide to about 2cm by 0 .5cm7 but the majority of the pieces somewhere in between.
I’m really surprised about how emphatic those designers were about not using gravel mulch, and their reasoning. Having mulched with gravel in many jobs, and at home, for 20 years now, I’ve never once seen any negative outcome that could be interpreted as the result of heat retention or increased evaporation.
In fact, anything that slows the movement of air over the surface of the soil, an breaks capillary action, will assist in retaining moisture. The fact that many gravel gardens end up being virtual seed-beds (for weeds as well as desirable plants) is proof of this. And I’d argue that while the surface of the gravel gets hot, the soil beneath it stays cooler. Of course I have no proof of this beyond my own experience.
But I think the use of the word ‘mulch’ muddies things a bit. If I use gravel as a surface, my motive isn’t primarily about mulch. It’s about a consistent surface against which plants look particularly good, and through which plants can puncture, and also to remove the interface between the path and the garden bed. That it also acts as ‘mulch’ in terms of water retention is purely a side-benefit, to me.
If I use an organic mulch, I never, ever want to see it, and it’s there for pure function ie water retention and weed suppression. It’s never really occurred to me to think of it as either/or, as if they’re interchangeable
(then, just to make things more complicated, one could add the argument that William Martin presented, from experience in his own incredible garden, Wigandia, in Mortlake, that organic mulches usually caught and captured water near the surface, resulting in much more shallow-rooted plants. Again, no science, but it’s always good to have our assumptions challenged)
I only mulch my garden paths with wood chips. The rest of the garden is never mulched.
I’ve been using Neutrog’s whoflungdung mulch and it is fantastic. Really expensive compared to many other mulches as it is biologically activated and inoculated with chicken manure, so I mainly use it on my roses and dahlias. It helps retain so much moisture and has kept everything looking really healthy despite the Queensland heat!
I am a big fan of gravel mulches ( less than 10mm size) in the UK. I reckon they do help a lot with moisture retention and cannot be a cause of heating the soil. I find it hard to follow the garden designers’ argument that gravel would contribute to heating the soil. Does sand do that on a beach? A good thick layer can only reduce evaporation and help to cool the lower layers, surely?
In relation to organic mulches, Dr Peter May (soil science expert, ex-Burnley) posted this on my instagram account in relation to a query about what type of mulch might be best “consensus is that 75 mm of mulch is good and in terms of water conservation it doesn’t matter much what mulch. Are you mulching vegetables or ornamentals? How are you irrigating? Are you irrigating? These will all have a bearing on your final decision. If you want long lasting organic mulch then woodchips best. There is growing support for gravel mulches though.”
I am very interested in gravel mulches and currently experimenting. Somewhat influenced by Olivier Filippi trial garden in Southern France and his books (just started reading the latest – Bringing the Mediterranean into your Garden). His uses mainly calcareous (lime/chalky) gravels grain size of 10-30mm, 10-12cm; and says the absence of fine particles makes it harder for weeds to germinate although some plants seed freely (although this is in combination with his particular approach to soil preparation). I have used 14mm Tuscan topping with fines washed out (long story – could not get aggregate in 14mm and wanted to blend with path), but about to add additional gravel – I think I will try a mix of 14mm Tuscan topping and 20mm Tuscan aggregate (smallest size available in this area for purposes of matching colour tones from quarry). A little bit of a risk with the fines but they have not crusted to date, so will have to see. A client has used 30-40mm pebbles and this seems to be working well. I agree with Jacquie that moving/planting is complicated with gravel but on other hand having it mixed in soil is probably ok. I think the larger sizes actually may be easier to move aside for planting. Another inconvenience is getting rid of leaf litter from the gravel. Will be interested to hear any discussion of gravel mulches at Mediterranean conference in Castlemaine in March (unfortunately booked out and wait-listed) – Michael McCoy, Dr Peter May and David Glenn will be presenting.
Thank you everyone for your thoughts and experiences. It seems there is a diversity of opinion – which I always take to mean there are many “right” answers! I am going to continue exploring gravel “mulch’ – I especially like the ability this provides to have no boundary between path and garden bed.
In terms of organic mulch, I hear on ABC saturday morning garden show the idea of a mulch made up of organic compost (potentially recycled green waste) and wood chops (e.g. 14mm pine bark) – 1/3rd to 2/3rd ratio. I have been using this and find it good in terms of allowing water penetration and it has attractive (hardly noticeable) appearance; its downside seems to be that it breaks down quickly (an upside actually I suppose as the organic matter is incorporated into the soil) and needs topping up regualraly. It is probably a bit light on in suppressing weeds……..
Straw mulch is the best organic mulch for vegetable gardens I think.
I’m a bit late to the party on this topic .I’m in southern Tasmania and have always sworn by gum bark mulch (Usually double cut) ,i have resisted using non organic mulches such as 10-20mm pebbles for a few reasons -1 .Its commonly used here on very basic gardens and is not to my taste visually 2. The hassle of moving it away when adding new plants so as not to mix it in to the soil .3. Cost and extra weight of spreading it .
However my thoughts on this are changing ,i am now considering switching over to a 50mm layer of limestone gravel as it is known as here which is basically crushed limestone to particle size of less than 10mm.I dont think it will cause to many issue as far as heating the soil goes and due to its light colour will likely reflect heat away.