Gardening
Q & A
Uses for old potting compost
'Is there anything useful I can do with old potting compost?' asks a reader, read Val Bourne's advice
Yes and No is the answer. Old potting compost cannot be reused for seed sowing or propagating unless it has been effectively sterilised because it might contain the eggs of insects (like vine weevil or flies), or pathogens that might lead to disease. Unfortunately sterilising soil involves high temperatures and it’s smelly - rather like fermenting hops. So this is not very viable for many home gardeners, although there are sterilisers on the market.
You can recycle friable loam-based composts (often sold under the recipe name of John Innes) by adding them to the ground to improve soil structure, or using them as a moisture-retaining, weed- suppressant mulch. If you add a fertiliser to the loam-based compost you can also pot up mature plants with it. I have a spent-compost heap close to my own shed and greenhouse which I use to pot up divided perennials.
Peat-based composts are impossible to rehydrate once they dry out. They can form dry balls in garden soil so you need to take care how you use them. They are not as easily recycled as loam-based products which contain sand, silt and clay.
Avoid peat-based composts
Gardeners care about the environment and extracting peat is known to be damaging to plant species and habitat. So we should look at other forms of compost in an attempt to find on good enough to replace peat.
New legislation
By 2010 new EU legislation will demand that 90% of composts have to be peat-free. So we gardeners will have to find a product we can trust because sowing and growing in poor composts leads to frustration and failure. Coir compost, made from coconut husk, has not been a success - but help is on the way.
Which Gardening? Survey Jan/ Feb 2009
Gardening Which? do an annual compost trial every spring. This year, with the new legislation in mind, the team looked at nine peat-free alternatives. The results were published in their Jan/Feb 09 issue.
New Horizon composts did particularly well in their trial.
- The New Horizon Organic and Peat Free Growbag scored 74%
- New Horizon Multipurpose Peat-free Compost scored 58%.
Both products are widely available and both contain composted bark, timber residues, green waste, limestone and nutrients including hoof and horn, rock phosphate, Vinasse and bonemeal.
What can gardeners do?
The scores of these two were still not as high as Homebase’s Multipurpose Compost (which contains 85% peat). It scored 83%.
Why the higher score?
Light, airy peat-based composts have the edge on peat-free ones because they are more suited for seed sowing. Seeds appear quickly and grow away well.
But if you add 25% vermiculite to your peat-free compost your performance will be improved greatly and you shouldn’t notice a difference between the two.
Gardening Which? gives the following pointers about buying compost
1. Buy fresh compost. Avoid any bags with faded labelling or broken packaging, or any that are water sodden.
2. Store compost in a dry place.
3. Water seeds little and often.
4. Feed seedlings after 4 - 6 weeks by adding a slow release fertiliser when you sow. ( Osmocote is ideal)
Green waste systems
But the real change is in the way local councils are dealing with green waste. Many are now collecting and composting green material from gardens and sterilising the product and then selling it on, rather than putting it into land fill. Some councils are selling it back to their residents and others are supplying commercial companies.
So there is no reason why you couldn’t put spent compost straight into a green waste bin - if you want to. It will still be useful. Other areas of the country will soon be catching up - so if you don’t have a green waste bin one should arrive sooner rather than later. Contact your local council and find out.
