I have three gorgeous compost piles rotting away nicely, and in June I took some material from one of them, spread it on my raised beds and also potted up some cuttings. We went away for a week in mid-July. There was a lot of rain. When we got back, the freshly composted areas were thriving with tiny nettle plants. I had my work cut out removing them, and they are still popping up. Is my compost doomed? I didn’t have this problem last year. G Loughman, Co Kildare, Ireland
There are two possible explanations for the appearance of nettle seedlings in your freshly composted areas. The first, which you suspect, is that your beautiful home-made crumbly compost contains nettle seeds, which then happily germinated once they were given the right conditions (a combinations of heat, light and moisture).
This isn’t unusual. Unless a compost heap is what’s known as a “hot heap” where the core temperature rises above 55 degrees, it won’t kill any weed seeds that may have inadvertently arrived in it as part of garden waste or even on the wind.
Bringing a heap to this temperature can be challenging. Alternating layers of carbon-rich material (such as autumn leaves, cardboard, paper, straw) and nitrogen-rich material (such as fresh grass clippings, vegetable peelings, leafy garden waste, fresh farm manure) is part of the secret, along with some sort of covered sides to keep the heat in and a thick insulating layer of old carpet, straw, or cardboard on top, capped off with a sheet of plastic to keep rain and weed seeds off it. Turning it also helps by boosting the process of healthy decomposition. Some commercially manufactured compost bins, such as the Hotbin (quickcrop.ie) help to make this process a lot faster and easier.
The other possible explanation for the appearance of nettle seedlings is that the seeds were already present in the soil, but the subsequent increase in fertility caused by spreading the compost encouraged their germination. Nettle growth is in fact a good indicator of a fertile, nitrogen-rich soil.
Either way, I wouldn’t be too concerned about using your home-made compost. An oscillating hoe will make short work of any baby nettle seedlings so long as you catch them early. These can then be raked up and thrown on your compost heap, where they’ll add a range of beneficial plant nutrients to the mix. Alternatively, you could make nettle tea out of them by soaking them in a sealed container with the leaves covered in water. Diluted down to the colour of weak tea and used as a foliar feed, it has a fantastic effect on plant health.
But if you’re determined to rid your existing heaps of any possible weed seeds, then I’d suggest creating a new heap as described above and gradually incorporating your old compost into it as you build up the layers. To make sure your new heap gets hot enough, use a compost heap thermometer specially designed for this purpose, which comes with a long probe that you stick into its centre (stockists include quickcrop.ie and fruithillfarm.com).
