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Thrifty uses for your old compost

Make the most out of this season's compost

Thrifty gardeners hate waste and bags of composts are no exception. 

For gardeners one of the biggest expenses of the year is the bags and bags of compost that are needed for pots, hanging baskets and growing tomatoes and cucumbers in the greenhouse. So Which? Gardening have come up with five great ways to get the most out of any compost you have bought. 

Which? Gardening compost expert Adele Dyer says: ‘By the end of the season the fertiliser you have added to the compost you used in your pots will have been used up, but the compost itself will still be good to use next year with a little care.’ 

Remember to use up any unused or part-used bags. Compost stored for a long time, especially if it has got too hot or too wet, will start to break down in the bags and the fertiliser in the compost could be released. If this happens your new, young plants may be harmed by acidic conditions. To find out more about compost, look at our reviews. 

1. Replant your pots

Which? Gardening tests have found that you can get more than one season out of your compost, so you don’t need to bin it at the end of the summer. We tried replanting pots with flowers and vegetables, and by adding a little slow-release fertiliser we kept them going for two years. To find the right one for you, take a look at our Best Buy slow-release fertilisers

Take out the roots of your old plants unless they have very fine roots that will rot down quickly. Use a hand fork to uncompact the compost and add the fertiliser. While you do this keep an eye out for any C-shaped white grubs in the compost. These might be vine weevils that eat the roots of your plants, which will kill them. If you find any, discard the compost. If the compost is vine weevil free, then you will be ready to add your new plants.

2. Grow your greens

Some plants need less fertiliser than others. We found that lettuce and other leafy crops such as spinach and pak choi are less hungry and so are good options for growing in old growing bags. Many bulbs also need very little or no fertiliser and so are great for using up leftover compost. 

If you are growing fruit and veg such as tomatoes and courgettes that produce a large crop, you will need to top up with a liquid feed, such as a tomato feed. There are rich in potassium which plants need to produce fruits. 

3. Steer clear of seeds

There are some thing that are less good in old compost. Seedlings and young plants are very susceptible to any nasties such as diseases, pests and too much fertiliser. 

Use new compost for seeds and young plants, to make sure they will get the very best start in life. To help you choose the right one, we have compiled a guide on how to buy the best compost

4. Fill raised beds

You can use it on your veg plot to bulk up raised beds and create the perfect growing environment for all your crops. Dig it in or spread it over the beds once the ground has been cleared and leave the worms to do the hard work for you. 

Whatever your soil type, it needs organic matter which helps to keep it healthy. Organic matter is any bit of a plant that used to be alive but has now started to rot down. This includes anything you would put on the compost heap, such as grass clippings, pruning and veg peelings, but also the contents of most compost bags. Which? Gardening has loads of advice on how to get started with making your own compost

Organic matter helps the soil to keep hold of the nutrients that plants need to grow and flower. It also help it to hold onto a good amount of water while at the same keeping the soil open and crumbly, so excess water can drain away and air can get down to the roots.

5. Mulch your beds

Compost works brilliants as a mulch on your beds. Not only does it add organic matter, but it also helps to keep moisture in the soil and, perhaps most importantly, it helps to keep down the weeds. Bagged compost is especially good at deterring the weeds. Because bagged composts are all made in huge heaps that reach a minimum of 65 degrees, all weed seeds that might have been in the compost are killed. 

Of course, if your compost is smelly or slimey, you will need to think again. In this case, add it to the compost bin and mix in well with plenty of harder material, such as shredded cardboard or straw. This will help the heap to regain its natural balance and break down to sweet-smelling compost that will be perfect for improving your garden soil. 

More on this…

  • For pots, read our reviews of Best Buy compost for containers
  • For seedlings and plug plants, see our Best Buy composts for raising young plants
  • Find out how we test compost
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