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Composting Leylandii, Conifer and Pine Needles HOTBIN


Some composting advice state: ‘do not compost leylandii, pine needles or other conifers as they create acidic compost and soils’.  Below we set outline whether this is a myth or fact.

Will leylandii compost in the HOTBIN? Yes.

Leylandii and pine needles can be hard to compost – but they obey the same rules as other wood based material. The speed at which they compost is linked to the size of the bits (surface area) and temperature (cold, warm or hot). Where it would take 2-5 years to cold compost them at 10C, hot composting at 40-60C it would only take 1-3 months.This explains why some people say report compost in 30 days and others that they still have leylandii in their heap after 5 years.

Small shredded pieces and pines needles will compost quickly in the HOTBIN – typically 90 days.   Do not add large non shredded bits of conifer to your HOTBIN – they will just come out the other end partially composted.

Conifers create acidic SOILS that kill plants?  Myth

Acidic soils are formed over millennia by weathering of rocks and minerals and the close interaction these minerals with organic matter that falls on the ground. Over time these reach equilibrium and you get soils that vary from acidic, neutral and alkaline (see soil classification). Most conifers, pine, leylandii and evergreen species grow in cold temperate climates on mildly acid soils – this is their natural habitat. The soil is acidic by location.

“You cannot grow anything under a pine tree due to acidity”

Most gardeners agree it is hard to grow anything under a pine tree. Research studies have shown there is no significant change in soil pH after years of adding pine needles to soil (ref Manynard et el). There are many acid loving plants – these would colonise the soil below the trees if acidity was the only problem – see below.

“CONIFERS release acidic chemicals when they fall to the ground”

True – but misplaced relevancy. Pine trees (like all woods and plant life) contain organic (carbon based) chemicals that chemists would class as ‘weak organic acids’. Plant life is made up of hundreds of these chemicals – turpenes, phenols, even the smell of ‘pine trees’ is an organic acid.

Organic acids are broken down during composting and return back to carbon dioxide. Think of it this way “acids in a pine needle are just bacteria food to be eaten”.

(It might be possible that “acid rain” that fell on many Scandinavian conifer forests has got muddled into this. Acid rain is caused by sulphur dioxide and nitrous oxide from coal burning power stations reacting with water to form weak sulphuric and nitric acid – which then kills the pine needles).

Lack of plant growth under pine trees is more likely a combination of three factors. A) Pine needles create a thick mulch layer (we define mulch is a dense layer of actively composting material). The bacteria in the mulch rob the soil below of nitrogen and hence prevent plant growth – typically mulches are used to prevent weed growth). B) Conifer roots are shallow – they out compete other plants for water and nutrients so plants struggle to grow around conifer roots. C) Conifers create dense shade which makes it difficult for plants to grow. All these factors create the ‘no growth zone’ rather just acidity.

“Leylandii are acidic – I get a SKIN burn when I cut the hedge”

Many people suffer skin rashes when leylandii clippings touch bare skin. The rash is a reaction to the weak organic acid – it is a ‘burn’. Some skins will be more sensitive than others – it is wise to wear gloves and clothes that cover bare arms and legs when cutting leylandii hedges. (Please see above – these acids are broken down in the soil via composting bacteria).

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