1 July 2012

The Poignancy of Restricted Roots

drawing of roots in a plant pot
At last, Garden 65 becomes the go-to blog for gardening tips ... !

The last post contained some useful information about a garden pest. Today’s practical advice is about growing plants in containers.

Did you know doubling the size of pot in which your plants are grown increases their size by about half again? In other words if you repot into a bigger container the plant will grow more stems and leaves, and this will keep happening every time you give them a new home. I suppose that’s obvious really, but now it has been scientifically verified by scientists, and they think they know why.

A meta-analysis of root related literature from the last 100 years has been done recently by a team lead by Hendrik Poorter in Germany. The conclusion was roots that are restricted in their growth send a signal to the rest of the plant to slow the rate of growth. And this process occurs in every kind of potted plant from little herby ones up to trees grown in containers.

The researchers looked at some of the more obvious problems caused by small pots, such as:

  •  Many plants in small containers can be grown close together and consequently the amount of light reaching the individual plant is reduced
  • Less nutrients available*
  • Reduced water holding capacity*
  • Temperature. Smaller pots get hotter faster. Experiments show a 5C temperature difference in 0.19L pots compared to 1.9L ones

*  However plants grown hydroponically with abundant water and nutrients, but restricted roots, still demonstrate reduced leaf and stem growth

But they found in their inimitable scientific way that these factors did not produce a mathematically significant change in plant growth.

The final suggestion of this research is that it is ‘root impedance’ itself that is the causal mechanism. An experiment showed that within 10 minutes of increasing root impedance leaf expansion rate reduced (what sensitive instruments they must have!).

Using MRI scans to look at how roots use the whole space allowed them within the pot the experimenters saw a consistent pattern, whereby the plant was sending out roots to the far limits but not using the inner soil. Generally 20-25% of root biomass was found in the inner part of the pot, and 50% in the outer 4mm next to the pot wall.

Root growth showing outer 50% in blue

It is as if the plants are searching for the edge and once it is found sending a signal to slow growth up above. Quite poignant really. Apparently when Hendrick realised this he repotted all his house plants.

"I thought, you poor guys, what have I done to you?"

Although I suppose we have to trust the scientists with their analytical skills and super-dooper measuring machines I’m personally unconvinced by this idea. There must be more to it than that. In my experience for example the Heucheras I grow in pots are bigger and healthier than those who have the freedom of all the soil of the flower beds (and believe me, it’s not because I feed them). And how many times have you dug up a plant you bought from a garden centre a few years ago and found the roots haven’t broken away from the circular pattern they developed in the pot? Life, as we all know, is complicated.