Some home truths about DIY fertilisers

I see them every day on my social media feeds – the amazing “home remedy” fertilisers that promise to dramatically boost the growth of houseplants. Everything from leftover pasta water to banana skin – even whole eggs – are touted as all-purpose elixirs. But how legitimate are these claims?

Let’s start with pasta water, which contains starch that leaches out during the cooking process. It is claimed that when cooled this starch solution is a plant fertiliser the effects of which verge on the miraculous.

What is often glossed over, however, is that one other thing pasta water can contain quite a lot of is salt. Depending on how much you use (Nigella Lawson says pasta water should be “as salty as the Mediterranean”) and how often you eat pasta (one of Britain’s favourite dishes), that’s potentially an enormous amount of salt accumulating in your houseplant pots. Just as in our own bodies, an excess of salt can dehydrate plant tissues and can as a result hamper their growth, even eventually kill them.

Even if you ditch the salt, starch is made up of carbon, hydrogen and oxygen, but plants mainly require nitrogen, phosphorus and potassium as fertiliser, so the nutritional benefits of this liquid to plants seems more than a little shaky.

Something similar applies to banana skin, which despite often being touted as being rich in nitrogen and potassium, in reality is only likely to contain a fraction of what any bog-standard plant food provides. If people feel the effects of banana skin really is “unbelievable”, I think we need to invent a new word for how effective actual fertiliser is by comparison.

As these are both products that would otherwise be disposed of, you could argue that you’d have nothing to lose, right? Well, despite being low in the nutrients that plants need, starch is a great fuel for bacteria and fungi, including potentially pathogenic types. So pouring starch-rich water on your plants is probably not a great idea, especially as mould growth is the perfect food for fungus gnats, little black flies which can soon fill your home.

With banana skins the advice is to let them rot down for weeks in water before dosing this round your living-room plants. Much like the idea of burying a whole, raw egg under the root ball of each pot plant, this is probably only a good idea for those who don’t have many friends or family, or indeed a great sense of smell.

The bottom line? Compared to commercial houseplant fertiliser, which not only has been specifically formulated to plants’ needs, but is easier to use, with little to no odour, homespun versions are likely to have more downsides than up. I’m all for horticultural thriftiness, but fertilisers are one of the cheapest aspects of gardening, especially with the tiny amounts you need for houseplants. So save your time and energy for enjoying your plants rather than tinkering with home remedies of little value.

Follow James on Twitter @Botanygeek