A container garden can be a delight, but it can also be expensive – the pots themselves aren’t cheap. If you have a water meter then you have to factor in the cost of keeping your pots watered, and you need fertilizer as well. It is usually recommended that potting compost be replaced every year, and if you’re buying good quality peat-free compost then the cost starts to add up.
The good news is that potting compost can be reused, which saves money and the environmental costs of producing and transporting it. There are some things to take into account – the potential for pest and disease problems, and the lack of nutrients in ‘spent’ compost – but once you’re aware of those then recycling your compost is easy.
One easy way to cut down on your need for fresh potting compost is to use your spent compost as a base layer in large containers. As we saw earlier in this chapter, adding crocks or other bulky materials to the bottom of containers to reduce the need for compost can be counter-productive. But a layer of spent compost would be fine, and you’ll have no problems with weeds if it’s well-buried.
If you haven’t had any pest and disease problems, you could also try sieving spent compost to produce a fine seed compost – the low nutrient levels are then a bonus rather than a problem.
Perennial plants in containers can spend several years in the same compost. Every spring you need to remove the top layer and add some fresh compost, or mix in a balanced fertilizer, to add nutrients back in.
You can also reuse spent compost for different plants. A good example is tomato grow bags, which can be used for a second crop of leafy vegetables. Tailor the addition of nutrients to the new crop. Avoid growing the same crop in the same compost twice, as it can lead to a build-up in pests and diseases – it’s the same principle as following a crop rotation in the vegetable patch.
Spent compost can be recycled elsewhere in the garden, where the remaining organic matter content will be of use. It makes a good mulch for the borders (again, you may need to sieve or pick out any clumps of roots) and can also be dug in as a low-fertility soil improver. We’re moving on to soil improvement in the next chapter.
If you have a compost heap then one of the simplest ways to deal with spent potting compost is just to add it in. (Another good reason not to add non-compostables into your pots is that you don’t have to sift them out when you add the spent compost to your composter later.)
Any plant roots will rot down, the composting process will help deal with pests, diseases and weed seeds (although it only does so completely in a very hot compost heap), and the remaining organic matter will become part of a new batch of compost with fresh nutrients. You can then use it in your own potting mixes next year, and reduce the amount of potting compost you need to buy.