What a load of compost

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At this time of year, I could do with the odd 30-hour day! Suddenly, everyone has realised that it’s spring and they’re in the mood to garden (or at least talk about it). As well as giving lectures during the day, I’m becoming busy in the evenings again, as there are lots of gardening clubs and horticultural societies in my area who have asked me to give talks and I enjoy it so much, I never seem to say no.

Interestingly, this year they’re all asking for similar topics, which is unusual – although much easier to prepare for! More people than ever are growing plants in pots and containers, with growing bags the most popular for all sorts of vegetables, and although the growing bag concept is around 40 years old, the rules are changing. Moss peat was the universal bulk constituent until about 10 years ago, when the environmentalists started to pressurise companies who were harvesting peat. This has brought peat-free growing bags and composts onto the market, but the trouble is, they’re all different. This is where I come in, trying to deal with the questions like which is the best? What can I grow in them? How much water do they need?

One problem arises if you try to compare the new compost with peat, because many of the new composts look like moss peat (brown and fibrous), but don’t perform like it. Trying to explain to an audience that a material that looks like one they have used for years is so different in its characteristics is never easy, especially when some have already decided that “this new stuff is no good”. To make matters worse, some of it is, at best, unreliable. At worst, it’s next to useless. The performance also varies from season to season, depending on how it has been composted and prepared.

My experience last year was no exception. I chose a leading brand of peat-free compost, sowed seeds into it and got an excellent germination. The trouble was that none of it was the seed I sowed, the seedlings were all weeds. This was a perfect example of a compost that either had not been left to decompose for long enough or the process did not generate enough heat to kill off the weed seeds. If the heat generated didn’t kill the weed seeds, it’s not likely to have destroyed any pests and diseases that were lurking around in the material either. Obviously, for propagating young plants from cuttings or seeds, the more sterile the propagation compost, the greater the chance of success and mucky compost is a definite no-no.

So then the audience ask if they can make their own compost. Immediately, I have to explain that there are two distinct types of compost. Bagged compost, bought from a retail outlet, is supposed to be relatively uniform, with bulk constituents such as peat, loam or composted and blended green waste, and added fertilisers to feed the young plants as they grow. Garden compost is pure green waste (such as shredded prunings, lawn clippings, and kitchen vegetable waste), but it’s variable content means it’s no good for raising plants in. It’s usually stored for a period of time to decompose, when it can be used as mulch or incorporated into the soil to encourage worms, bacteria and other organisms, which improve soil fertility.

You know when garden compost is ready to use when you can’t recognise what went into it, but this takes time, because different materials break down at different rates. Hard, woody materials like prunings take many months to rot, while soft fleshy plant debris can decompose in just a few weeks. Composting is always more rapid in the summer and autumn, when conditions are much more uniform and not too hot or too cold.

The best part about talking to other gardeners is that no matter what information I can pass to them, I always leave having learned something new myself.

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