I have a lot of plants in pots and troughs that have been doing very well till now. The problem is that a number of them seem to be losing the colour from their leaves what could cause this?
Where exactly are the leaves yellowing? At the base, between veins or in spots? Yellow leaves can be caused by a range of things from pests to environmental changes. You should examine the plants in question to see if you can find evidence of any pests. Otherwise it could be a case of too much / little water, lack of light... Maybe you could ask someone at a garden centre - they may even to offer you a remedy.