ALWAYS UNDERFOOT

Rather than being a fly on a wall, I would want to be a worm in my garden. I’d learn what tastes good, and what doesn’t, in compost.

I’d introduce myself to all the microorganisms underground and watch some mycorrhizal fungi grow. But best of all, I would be able to travel amongst and study roots and see how they grow and interact.

Roots are the powerhouse of every plant. Growth rate, plant health, fruit and flower production as well as disease resistance all rely on a robust root system accessing nutrition from the soil.

But we know so little about roots. We can’t see roots in action and no worm has told their secrets. But we do know some things.

We know that roots and the leaf area are in proportion to each other.

Prune the leaves off, and the roots will shrink and reduce down accordingly. Prune the roots and the leaf growth will be reduced. This is how bonsai works.

And this is also why we should prune a new bare-rooted tree upon planting. The nurseryman has trimmed the roots when he dug the tree to sell to you, and if you don’t prune the leaves back too, there won’t be enough roots to sustain healthy growth in spring. Trust me, the new growth will be stunted, prone to disease, weaker and slower than if you had been ruthless with the secateurs.

We know roots continually seek out nutrients.

That roots only grow as far as the drip line of the branches is a fable. The root’s job is to find nutrients, and if there are not enough under the tree, they keep on growing. Just ask a council worker – they can tell underground-triffid stories of poplar roots in drains 110m away. A root needing nutrients or water will keep seeking them until it finds a barrier, such as a lack of oxygen. Roots cannot grow where there is no oxygen or where the soil is compacted and hard to penetrate.

Roots, like leaves, are continually replenishing themselves.

They live and die. Old feeder roots, the fine ones, that have used up their surrounding nutrients and are no longer active, die off. But they will regrow when nutrients become available.

Some big trees send down a central taproot, a major anchor deep into the ground, mining nutrients and water from way below. And it can be way, way below – in dry countries going 30m deep to get water. This taproot grows and establishes itself first, before much growth is seen above ground. But once the roots are established, then and only then will the tree start to grow.

This means that a decent-sized, nursery-bought tree has usually had the taproot cut when lifted for sale or distorted and damaged in a planter bag. And it never re-grows the same. For these sorts of trees, such as hickory and avocado, buy small trees and plant out as soon as possible or plant seeds in situ.

Roots can’t take up nutrients by themselves.

They rely on symbiotic mycorrhizal fungi to break down nutrients into a plant-available form and transpose them into the root cells. Some plants even have special mycorrhizae symbiotic just to them – so transferring a bit of soil from home ground is a good idea when acquiring plants from friends.

The variety of roots can have a huge influence on the tree.

In grafted trees (where the tree top and different roots are joined together to form one tree) the rootstock can influence aspects such as disease resistance, bearing age and branching unions.

This is why most New Zealand citrus is grown on trifoliata rootsock – it makes our citrus more resistant to cold and phytophthora disease.

But there is a lot we don’t know about roots.

Mainly because we are not a worm. How fast do they grow? When do they grow? Seasonally? When water is available? Only when nutrients are available or needed? When the leaves are growing? Before the leaves can grow? How long do they live? How do they interact with organisms in the soil? And what happens when they are flooded or dried out?

Analysing roots is difficult for us above-ground dwellers. Most roots are so fine, the very act of digging them up destroys them, and any interference, well, interferes with their growth and therefore distorts their natural patterns.

I am not the only one wanting to be a worm. UK’s Kew Gardens has a rhizotron – an underground laboratory with windows – but that in itself is interfering.

Technology has got smaller and the Australians are using minirhizotrons to monitor root production and turnover in almond orchards. Minirhizotrons is a lot of syllables for a clear tube inserted into the ground with a camera inside. This takes images of the roots at different depths over a period of time. We can watch roots grow – and die.

And hopefully learn a lot more about what goes on right under our feet.