The right compost can make all the difference to container-grown plants. Learn about the three basic types of compost and how to tailor them to specific plants’ needs, in our No Fuss Guide to choosing compost, with David Hurrion, BBC Gardeners’ World.
Choosing compost for propagation – transcript
When you’re raising your own plants, whether it’s from seed or cuttings, you want to make sure that you get the best possible results. And one of the keys to success is making sure that you choose the right compost from the start.
Now, there are some basic composts out there, but there is such a boggling range that it’s easy if we just break them down into three basic types. The first is loam-based compost. That’s compost that’s made from well-rotted turf and also with a little bit of peat added, but then with some coarse horticultural sand thrown in for good measure. This is really good for raising things like pelargoniums from seed that need some free-draining compost to really get them going. But it’s also good for hardwood cuttings and semi-ripe cuttings, that are going to be in the compost for a long time, so need something that’s firm to hold them in place, but also free-draining, so that they don’t rot.
Then we move on to peat-based compost. Now there are lots of these available. Most of these are available in garden centres and DIY stores and sometimes there’s very little else to choose from, to be honest. And the reason that peat has been so popular in the past is that it has a very fibrous nature and it holds onto lots of moisture. If I squeeze that, you can see I’m squeezing out the moisture and that means it’s really good for things that need moisture over a long period in order to germinate. So it’s good for lots of the annual seeds, but it’s also good for softwood cuttings, that want the water to enable them to root really quickly. Now, one of the biggest problems with peat-based composts, other than their lack of environmental credentials, is that they sometimes hold on to too much moisture, which means that you can get a green algal slime forming on the surface of the compost, which looks both unsightly, but it also traps even more moisture. But also, that can cause your seeds and your cuttings to rot off or damp off. But some of the new peat-alternative composts overcome that problem by having a lot coarser structure. Lots of them are based on a mixture of bark and well-rotted green waste. And most of the new formulations of peat-free composts are fine enough to be really good for growing things like seeds and cuttings. And they are useful for a whole range of different seeds and cuttings, particularly things that like drier, open, composts. They maintain their moisture, you do need to water them perhaps more regularly, but they maintain their moisture and their aeration reasonably well. So the latest ranges of peat- free composts are much better for germination than the older ones – they’re much more moisture-retentive and yet free-draining.
But the thing about propagation is that you can never find one compost that suits all aspects of seed sowing or cuttings. And so what I like to do is try and formulate or specially blend or add to my basic compost so that I can tailor it to get the best results. So, for things that like freer drainage, it’s always a good idea to add things like this perlite. This is a volcanic material that holds onto a little bit of moisture but actually is really good for encouraging good drainage. It holds the structure of the compost really open in the same way that adding coarse grit would do. Now coarse grit is a much heavier material than perlite, so it depends what you’re propagating. If you’re growing things that are going to be in the compost for a long time, things like hardwood cuttings, for example, then adding grit to the compost will add more stability to it and hold the cuttings upright.
However, once you’ve filled your pots with compost, whether you’ve filled them with compost and sown seed on the top, or whether you’ve inserted cuttings, it’s then a good idea to have something solid on the surface, to stop the water splashing the compost up onto the leaves of the plants and also to hold seeds in position, to help them germinate really well. Another thing that you add to the surface of the compost rather than mixing it in, is vermiculite. Vermiculite is used to cover very, very fine seeds just to hold it lightly in place without too much weight. It’s not mixed into compost because it holds onto too much moisture and can cause problems with algae. So choosing the right compost and tailoring it to suit the needs of your plants will make sure that you get the best possible results, time after time.