Top Three Most Surprisingly Difficult Materials to Compost

 With hot composting (see blog posts 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11), you stand as good a chance as any of successfully composting most kitchen and garden waste. Soft green waste (e.g. lawn mowings, cooked & uncooked kitchen scraps) are the easiest while hard woody stems/branches are the most difficult. Cutting/chopping the material before adding to the compost pile speeds up the decomposition process by disrupting cell membranes (allowing bacteria faster access to the fleshier more easily digested innards) and increasing the surface area which is the frontline of bacterial attack. That is why I shred garden waste and chop up kitchen waste before adding to the compost pile. 

Photo 1: Thermal Infra Red Image of HotBin Contents (including 3 probe thermometers)

Some everyday (eggshells, some teabags) and not so everyday (avocado skins) items can make it through the hot composting process in a still recognizable form. With teabags, the contents (tea leaves) will have successfully composted but the bag itself may remain largely intact. Either discard the bag or recycle it through the next hot composting process. Eggshells are predominantly calcium carbonate so cannot be composted down - they are, however, a valuable source of calcium which is an important plant nutrient. And, finally, you could be a little less posh and not eat avocados!?! While avocados are exceptionally nutritious, tasty and versatile, that comes at an environmental cost that may not be sustainable. As a general rule, we no longer buy avocados for home use though I must admit, that is due, in part, to pinpointing exactly when those 'ripen at home' avocados are perfect for eating! I did enjoy a recent vegan breakfast at the Rocket Kitchen Cafe in Hereford that included a deliciously ripe avocado.

Photo 2: Vegan Breakfast at the Rocket Kitchen Cafe 

So enjoy your avocado as a treat rather than an everyday food item like Posh Spice.

Back on topic, the three items that regularly appear in my garden waste pile (almost exclusively from neighbours!) that are difficult to compost.

1. Dried palm leaves

My next-door neighbour has a palm tree of some description ...

Photo 3: Neighbour's Palm Tree

... from which I receive a fairly regular supply of long dry brown leaves. I chop these into 10 cm lengths with sharp secateurs before processing them through the shredder.

Photo 4: Palm leaves after pre-cutting with secateurs

The plant fibres are tough and run the length of the leaves and so 'shred' in the same direction. Furthermore, they have poor rewetting properties essential for the bacterial decomposition process - on the other hand, they make excellent building materials! These materials will eventually break down into usable compost but will need at least 3 cycles through the hot composting process. As a general rule, shredded palm leaves are added in small quantities - say up to 10% with plenty of green waste.

2. Clematis Woody Stems

Back in March (2023), I took delivery of around two cubic metres of old-growth woody clematis stems from a neighbour. This was the first batch of 10 bags ...

Photo 5: First batch of woody clematis stems

... followed by a further 15 bags. We have our own vigorous clematis which grows over the summerhouse; see below ...

Photo 5: Clematis "Snow in Summer" (Aug 1st 2023)

... and which is cut back hard every year.  The cuttings are easily composted because there is no multi-year growth to deal with. Our neighbour clearly had not cut back her clematis for several years and had finally decided it was time to get rid of the resultant unruly mess.

I had two issues: (i) the fibrous clematis stems were dry and difficult to shred, and (ii) even the shredded stems were not easily wetted (see above, re palm leaves). Stone Age people apparently used clematis stems as ropes which is understandable considering their toughnesss and durability.

Photos 6 & 7 were taken before and after shredding the clematis. The shredded material varies from dust to fairly long fibres depending upon the sharpness of the shredder blade.

Photo 6: Tangled Clematis Stems before shredding

Photo 7: Clematis after shredding

Processing old-growth clematis is a new experience for me. I add it to the hot compost pile in small amounts (up to 20%) mixed with plenty of moist greens. Even after a few days, it still looks dry and unaffected by the hot composting process. We will have to wait and see how it comes out at the end of the composting period. I suspect it will need to be recycled through extra composting processes.

3. Thatch and Moss

We do not have a lawn so, until recently, my 'supply' of thatch and moss occurred once a year in Autumn when a neighbour scarified his lawn. It was quite a big lawn so I received about 120 litres of the stuff. Since this neighbour moved a year or two ago, I no longer have to process thatch & moss - thank goodness!

Thatch is the layer of dead turf/grass just beneath the surface - it contains about 25% lignin (similar to wood) which explains why it is slow to decompose in the composting process.

Mosses (and their close relatives: liverworts & hornworts) are bryophytes and have been around for 450 million years living through many climatic changes. They are tough and born survivors. By way of example, I use an electric weedburner to keep our paved driveway free of weeds. The weedburner claims to reach temperatures up to 600 ℃ which is sufficient to burn away common weeds (e.g. dandelions) but has little or no effect on the tiny liverworts that come up between the cracks. Unsurprisingly, the temperatures found in a hot composter (50-70 ℃) don't cut the mustard either.

In my, albeit limited experience, the scarified moss/thatch can have a detrimental effect on the operation of the hot composting process (e.g. HotBin). Thatch (high lignin content) & moss (tough resistant cell walls) do not add anything in the way of nutritional value for the composting bacteria to thrive and multiply and may have a negative effect by absorbing water (thereby reducing its availability for the composting process) and reducing the ability of the compost pile to self-aerate. You might want to find an alternative disposal method for your thatch/moss but if you must add it to the hot composter - no more than 5% by volume.