Windowboxes, in particular, can dry out quickly as a result of the combined factors of water loss, so making an effort to reduce these as much as possible will mean that your plants will stay healthier and you don’t have the chore of watering quite as much!

 Roots of growing plants competing for water resources

If you’ve got perennial or shrub-like plants in your windowbox, such as soft fruit bushes, there is no harm digging up the plants at the end of the season to chop back some of the roots and pruning the stems (a bit like bonsai, but not so extreme). This is also a good time to replenish the compost, which will probably need bulking up again with more fibrous, nutritious matter.

 Water loss from the bottom of the

windowbox

1. If you’ve followed my instructions for making your own windowbox you can line the bottom third of the box with pond liner, punched through with about half a dozen small holes to slow the flow of water out the bottom (The gaps between the planks higher up mean that excess water will escape through the sides to avoid any possible waterlogging). This particularly suits damp-loving plants, such as cowberry & cranberry that also require an acidic soil.
2. The liner can then be covered either with coarse twigs or polystyrene chunks to provide a ‘reservoir’ for the plant roots to reach down into and for the surrounding soil to soak up.
3. Make sure you use a good soil mix, including plenty of fibrous compost to cover the reservoir. This will absorb water like a sponge and keep it locked up until taken up by the plant roots.

 Water loss through plant leaves, particularly on hot or windy days

Try and pack in your plants (within reason), with taller plants at the back, graduating to shorter plants at the front. This will help protect the taller plants from strong winds and also create areas sheltered from the direct sun and wind. Grouping plants together also increases the humidity around the plants – all of which helps to reduce overall water loss. You could also drench leaves from above on hot days (out of direct sun)

 Water evaporating from the soil surface

The other thing you can do is to look at ways to reduce water loss from the soil surface by applying a mulch – This could be an organic material, such as grass cuttings, bark chippings or garden compost, or an inorganic mulch, such as gravel (good for mediterranean herbs that like particularly free-draining soil)

Early spring is an ideal time to mulch, before most plants start back into life. An organic mulch also has the advantage of feeding the plants at the same time.

Most plants are easy to mulch, such as the celery and bloody sorrel (right), whilst Mint can be mulched before it regrows (top left). Don't be afraid to put organic mulch on top of clump forming plants, such as Lemon Balm (bottom left)

Mintmulch
Lemonbalmmulch