How to tell if your plant is getting enough light.

A good share of the "sick" plants we are called to see are not sick at all. The lower leaves are yellowing, the plant drinks and wilts again a day later, and new growth has stopped. The owner has tried fertiliser, then a fungicide, then more water. What is actually wrong is that the roots have filled the pot and have nowhere left to go. The good news is that it takes about two minutes to check.

What root bound looks like from above

The signs above the soil look like a dozen other problems, which is why this gets missed. Water runs straight through and out of the drainage hole within seconds, because there is more root than soil to hold it. The plant needs water every day when it used to need it every three. Lower leaves yellow and drop. Growth stalls in the growing season. Sometimes a plastic pot bulges or a nursery bag splits.

The clearest clue is roots where they should not be. Look at the drainage hole. If roots are poking out, or have grown into the saucer, the pot is full. Plants bought in black nursery bags and never moved are the usual suspects. So are fast growers such as money plant, areca palm, peace lily and spider plant, which can fill a 6-inch pot within a year or two.

The two minute check

Water the plant lightly an hour before so the root ball holds together. Lay the pot on its side, hold the plant at the base of the stems, and tap or squeeze the pot until the root ball slides out. For a large pot, run a blunt knife round the inside edge first. If you see mostly soil with white roots spread through it, the plant is fine and the problem is elsewhere. If you see a solid mat of roots circling the outside, taking the exact shape of the pot, it is root bound.

Slide the plant out of its pot before you spend money on fertiliser or pesticide. It costs nothing and rules out the commonest cause.

How to repot it

Choose a pot one size up: about 2 inches wider for a pot up to 10 inches, and 3 to 4 inches wider for anything larger. Going much bigger leaves a mass of wet soil the roots cannot reach, and that soil sours. Tease the outer roots loose with your fingers. If they have formed a hard mat, make three or four shallow vertical cuts down the sides with a clean knife. It feels brutal, but it pushes the plant to grow new roots outwards.

The best months are February to March, and again after the monsoon in October. Avoid repotting in the April and May heat if you can. If it cannot wait, do it in the evening and keep the plant in shade for ten days. Expect a sulk for a week or two: a few drooping or yellow leaves are normal while the roots settle. Do not fertilise for a month, and water only when the top inch of the new mix is dry.

When a bigger pot is not an option

Some plants are already in the largest pot the balcony can take. For these, root pruning works. Slide the plant out, trim about a quarter of the root ball from the sides and bottom with a clean knife, add fresh mix and put it back in the same pot. Trim a similar share of the top growth, so the plant is not supporting more leaves than it has roots for. Most shrubs and palms handle this every two years.