Hi Sibe,

Because we are in rented property, most of my "garden" is in large terracotta pots. I find it difficult to be sure when plants need more water as I am worried about over-watering, which I know plants can die from. I once heard mentioned on one of Alan Titchmarsh's gardening programmes a moisture metre that you stuck in the soil and it told you the moisture content, but can't find one anywhere. Can you help?

Many thanks,

Denise Parkin.

Our Gardening Expert replies:

Hello Denise,

Many plants can die when their roots are too wet as these require oxygen to continue growing. Some specialised plants are able to cope with growing in very wet conditions. This is because they take in an oxygen supply from higher up the stem and transmit it down to the roots, which can therefore grow underwater. Most plants that we grow in this country, however, would have difficulty in such situations.

In practice though, you have little need to worry about plants in large terracotta pots except in certain circumstances. The first of these is if you have used garden soil, as opposed to potting compost, in the containers. Garden soil, apart from being too low in nutrients to keep most plants happy when their roots are confined to a container, is likely to be too 'claggy' in this situation. Provided you always use a good potting compost - and a John Innes one in preference to a soil-less one, most plants will be OK even if heavily watered.

Secondly, if the plant concerned becomes dormant over winter and its roots then stand in cold, wet compost this will encourage rotting. Again, some plants can cope with this, but not all, so it's generally a good idea to keep dormant plants on the dry side (this means slightly moist rather than actually being dry) until they start into growth, after which they can be watered as normal.

The last situation is when a small plant has been transplanted into a large container. In this situation, over-watering can prevent the plant's delicate young roots growing into the rest of the container, possibly resulting in the death of the young plant.

Generally speaking, provided the container is not over-large for the size of the existing root ball and provided you have used a good potting compost, there is less danger to your plants by over-watering. To make absolutely sure you should always 'crock' the bottom of the pot - covering the drainage hole(s) with broken plant pots or broken ceramic cups etc so that the hole does not become blocked with compost. As a further precaution you could always add extra grit to the potting compost mix.

As regards moisture meters, these can be useful and you should be able to obtain one quite easily for about £10 if you look in a large garden centre. In time, however, I'm sure you will be able to judge whether the plant needs watering as you gain experience. Don't forget you can always push your hand down the inside of the pot to see how wet or dry the compost is, or you may be able to judge it by the weight of the container.

For more helpful tips, see Container Planting, Window Boxes, Planting Tips and Watering Tips.

All the best,

Sibe