Notes from the Nursery Gardens - March

‘If there is one task that you do in the garden to improve it, it’s mulch’. Sage advice from a seasoned gardener early on in my horticultural career that has proved its worth. The benefits of an annual covering of a couple of inches of organic matter over bare soil at this time of year are many. Keeping the soil healthy is key to a thriving garden and most soil types can be improved by mulching. The activity of earthworms and other soil fauna improves the drainage on heavy soil as the organic matter is pulled down below the surface. The organic matter also helps flocculation in the soil, where the soil particles form clumps or aggregates, thus improving porosity allowing air and water to pass between them. It also increases the cation exchange capacity (CEC) which is basically the soils’ ability to supply nutrients for plant uptake. Win, win, win. On sandy free draining soils, the organic mulch helps retain moisture and on the surface limits evaporation. The mulch can suppress annual weeds, protect against soil erosion and all that soil science aside, is there anything more heartening than seeing fresh green growth growing up through a nice dark mulch.

Now the mulch material itself. Good quality garden compost is the ideal. As gardeners we are constantly harvesting plant material from the garden. Using up nutrients from the soil, all that plant growth that we snip, cut, mow, weed and harvest can be converted back into usable organic matter that we know has a raft of benefits for the health of our soil and return some of the nutrients used. There is an art to composting. Many have become obsessed with the subject and the more that is understood about the process the more fascinating it becomes. A recent scientific study found that Mycobacterium vaccae found in compost have a similar effect as anti-depressants, lowers inflammation in the brain and appears to help post-traumatic stress disorder, and are now trying to create a medicine from the bacteria. Compost actually makes us happy, who knew!

Hot and Cold composting are the two methods most used in our gardens. Hot composting is the best way to convert all that plant material back into nutrients and beneficials to return to our soil. It is a quicker process, and the heat of the heap (50-70 degrees) kills of weed seeds and pathogens. Now it’s all about the C:N ratio. Layering up green (Nitrogen rich) and brown (Carbon rich) material to a ratio of 1 part Nitrogen to 25 parts Carbon is the optimum. Maintaining moisture, heat and air in the pile of green and brown material is the key to keep it ‘working’. This is achieved through periodically turning the heap. A cover over the top to stop it getting waterlogged helps but add water if it starts to dry out. The downside to hot composting is that it takes up space. Ideally two bays about 1m3 each, so we can turn one bay to the other, are needed. The other point is to have enough material to fill a bay relatively quickly. Cold composting is just layering up material in a bin as and when and leaving it, removing the usable compost from the bottom. With less heat in the pile, weed seeds and pathogens won’t necessarily be killed and the process is a lot slower.

We are currently building a new composting area at the Nursery to create our own lovely organic mulch. I’d amend that sage advice to ‘If there is one thing that you do in the garden to improve it, its compost. It makes you happy.’