ZOMBIES ARE TRUE, THIS ISN’T

NO. 3 ‘YOU KAN GET ALL YOUR K FROM KOMFREY’

Many gardeners will tell you that comfrey – a hairy brute of a perennial, with deep taproots, a large spiralling crown of leaves and pinkish-purple bell-shaped flowers – is the Best Plant Evah for drawing potassium from deep layers of soil and storing it in its leaves. Grow enough comfrey, they say, and you can meet the K needs of your cropping plants by chopping up its leaves and laying them on the soil, adding them to your compost or steeping them in water to make a (very stinky) liquid feed called comfrey tea.

This isn’t strictly true. First of all, comfrey is far from being the only plant that stores potassium in its leaves, and it isn’t even the best: the leaves of horrible old stinging nettle contain almost exactly the same K concentration, while red clover shoots hold almost a third as much again.57

Comfrey does have potassium: its NPK ratio is 4:1:16,58 and you’ll find around 10 g K in the cut leaves of one mature plant. An established comfrey plant can handle being cut down twice a year without weakening it, meaning one plant can provide a potential 20 g K a year. Meanwhile, your super-hungry veg plants will need to absorb around 30 g K per square metre every year.59 That means in theory you should plant one and a half comfreys for each square metre of veg garden.

This seems doable until you realize that a mature comfrey plant will easily take up a whole square metre of space. In other words, to meet all your K needs with this feisty grower, you need to use 60 per cent of your site for a permanent planting of comfrey.

So by all means plant comfrey wherever you have room for it: it’ll grow in shady spots that are no good for veg, and it’s useful to have a lightweight, renewable source of K on-site. But you’ll still have to bring in potassium from elsewhere. Which means back to wood ash and – sorry – poo and pee.