Unlike formal lawn gardens, where you need not to worry about changing composts, the container garden require frequent or periodic replacement of composts in the tubs, pots, hanging baskets, window boxes, urns. Because the over time the composts deteriorate with drainage and aeration becomes poor.
As we know that small plants do good in small containers, but it is essential to pot on to the containers two times larger than the precious container before they become container bound. This doesn't mean that you put small plant in a large container, because the large amount of compost around its roots may cause them to rot. Yes, for large plants you can start off with larger container to accommodate them well. The ideal time for potting on is the early spring.
It is ideal for plants to repot them every year to ensure a fresh supply of compost to them. The ideal time definitely would be the early spring.
If you need to repot a large plant, you may need a help of other person. To begin with, place the container on its side and ask other person to tap the rim of the container with a wood block gently. In mean time, the first person pulls the plants softly and steadily until the root ball would slide out.
What you need to do now is to reduce the size of the soil ball by tapping away about 5 cm (2 inches) of the compost from the sides, bottom and top. You can also prune the roots as well. Cover the root ball of the plant with wet sacking to avoid drying out. Now rub off the container properly and wash it with water to remove all the traces of compost from it. Allow it to air dry properly. Once the container is dry, replace the plant in its container with fresh compost.
If for some reasons repotting is not possible, you can simply do the topdressing in spring. You need to remove about 2.5 cm (1 inch) of old compost from the container and put some fresh compost to fill the depth.
If you use your container for seasonal plants, i.e. like for spring or summer bedding, then it is recommended to replace the compost after every second year or so to keep its strength intact.
Hanging baskets are usually used for seasonal plants and it is suggested that change the compost at the end of each season before planting new plants.
The perennials and alpines are such species of plants which often get split or divided in the container in small portions. There is no need to buy the new plants in that case. In this case reviving the double walls can be enough to keep things under controlled.
Large plants or raised beds or borders may need no trimming for long time. Although they grow in their sizes over time, they don't need immediate consideration; instead you can lift and divide the perennials and groups the bulbs after every three or four years.