Plants with a hidden message…

Yarrow - good at accumulating zinc

by Alan Titchmarsh

Plants perform a lot of valuable services to mankind, but one of the strangest is by helping geologists and prospectors find deposits of mineral ores including gold, copper, or even uranium all round the world.   Most  ’indicator plants’ are uncommon wild species, though you might have a few growing in your garden.

But before you start digging up the lupins in search of buried treasure, the presence of an ‘indicator plant’ doesn’t guarantee there’s a fortune in minerals underneath. Some thrive in mineral-rich soils where other species can’t survive, but others simply grow more vigorously than usual or develop brighter leaf or flower colours in the presence of  ‘their’  favourite mineral. And since traces of minerals are often carried miles from their original sources, you need to be in-the-know geologically besides knowing your plants.

In Finland, a huge deposit of copper and nickel was discovered after a mining-engineer analysed the metal content of some birch leaves. In Missouri, USA, a plant called leadwort (Amorpha canescens ) thrives in soils near deposits of galena ore, from which lead is derived. In Nevada, an unusual species of milk vetch (Astragalus pattersoni) has helped prospectors find uranium deposits, whilst in Colorado a closely related astragalus species ‘indicates’ selenium.

And in Arizona, home of the original Gold Rush, modern day prospectors still look for the desert trumpet plant (Eriogonum inflatum) when searching for likely sites to find gold. (It’s a very odd looking thing with a round flat rosette of basal leaves from which grow airy branching stems with inflated sacs at the joints).

In South America a particular aspen often grows close to deposits of zinc, while in Europe experts look for a rare yellow-flowered pansy (Viola calaminaria) whose flowers are brighter at high zinc concentrations, or a peculiar penny cress relative (Thlaspi calaminarium) which grows larger than usual where there is zinc in the soil. Copper-loving plants have also been successfully used to make big ore discoveries in the Chilean Andes. And in Britain both tungsten and tin have been discovered, over the years, by following plant clues.

 So far over two hundred plants have been discovered that have the ability to be of use in this way. Some species so good at storing up heavy metals that experts can extract quite useful quantities of valuable metals from the ashes of the plants. Ordinary yarrow (Achillea millefolium) is very good at accumulating zinc, and common horsetail  ’a ‘problem’ weed ‘ does the same with gold.

(The odds of striking gold in your garden are pretty remote  but what you will find in home-grown horsetail is silica, a fine sandy like substance which gives a handful of screwed up horsetail leaves the abrasive quality that makes them useful for shining pewter, or as a handy bio-Brillo-pad for cleaning pots and pans’ both tips used by medieval housewives).

Scientists in some parts of the world find the metal-accumulating properties of certain plants make a cost-effective way to clear polluted ground of cadmium, nickel, copper, zinc and lead, which can then be recycled from plant remains, if the concentrations make it worthwhile. A few species of plants can even ‘soak up’ spilt petrol or diesel from contaminated soil.

The early prospectors of a century or so kept their plant-secrets to themselves, but now they’ve paved the way for investigations by university research departments and commercial companies in several parts of the world. Who’d have thought plants would have so much tucked up their sleeves?