Unlike plants growing in the borders, which can send out new roots in search of food or water, those in containers are completely dependent on you for all their needs. Along with checking regularly through the season for watering and attack by pests or disease, annual maintenance is important if they are to grow well and remain healthy.
Weeds in the pot compete with the plant for water and nutrients. It may be necessary to remove the top 2cm of compost to make sure you get rid of both weeds and seeds.
Replacing the lost layer with fresh, sterile compost helps reduce the weed problem and, if the compost contains fertiliser, feeds the plant too.
If there is no food in the compost, you can add controlled-release fertiliser to last through the season. This only works properly if it is dark and moist, so fork it gently into the upper level of compost.
For a long-term planting, mulching the surface with gravel, slate chips or bark will suppress weeds and conserve moisture. This will need washing or replacing when you repot the plant.