So when should you prune? Well, it helps if you can identify what type of clematis you have and then prune according to the groups below:
If you are planting a new Clematis, regardless of which group it belong to, it should be hard-pruned in their first spring, which would mean taking each stem back to an outward facing bud about 15-30cm above the ground.
Before undertaking any pruning work, make sure your plant is healthy. Remove any dead, damaged, diseased and displaced material:
Group 1: are small flowering species, that flower on last seasons growth and have finish flowering by June. Every year you would undertake general maintenance pruning straight after flowering, cutting back the flowered stem by about half of their length.
Spring-flowering species like C.montana, come into leaf as they start to flower and winter-flowering species are evergreen like C. armando, so pruning will involve cutting back into the leaf-covered stems.
If the Clematis has been left over a few years without being pruned then you will need to undertake renovation pruning, this will produce re-growth and you can re-shape the plant. To do this involves cutting back the whole Clematis to about 15-30cm from the base of the plant, you will then not get flowers until the next year, but the plant will hopefully provide an abundance of fresh growth and larger, healthier flowers.
Do not over fertilise if you have just renovated the plant as it may be in shock and needs resting, if you apply fertiliser it will have to work too hard.
Group 2: Large flowering species that are Hybrids. They flower May - June on short laterals that develop from the previous years growth. Some will re-flower in summer on current seasons growth.
General prune in early winter or early spring just before they start to grow, removing any dead, diseased, damaged or displaced material, then look for strong, outward facing, thick buds to prune back to. (the buds are in pairs).
You only want to prune a 3rd of the growth, this will enable the plant to concentrate its energy into stronger shoots and flowers. After the first flowering in the year you can prune some of the stems back to buds, below the old flowers, this may give you a second flowering.
If the clematis is very crowded you can also reduce some of the congested stems at the base, and be mindful that if you prune too hard one year, the flowers will be smaller the next year but then will be back to normal size.
Group 3: Large flowering cultivars/small flowering cultivars and herbaceous perennials. These flower later in late summer and autumn and are the easiest to prune. They flower of current seasons growth and if left unpruned they would become very woody, with lots of new growth growing through the old growth and it will end up looking like a birds nest, with all flowers being produced at the top of the plant.
So to prune these clematis's you cut back the whole plant to about 25cm above soil level to a health but in late winter or before the new growth begins in early spring.
General maintenance would also be to keep the root system moist, especially in the first couple of years of planting. Keep the base of the plant well mulched, this will deter any hungry slugs.
A few diseases to be aware of:
Clematis wilt: usually affects the large hybrids, in summer, so I may look like the plant needs a good watering. It can affect some or all of the plant. Cut the affected areas and make sure you cut at least 30cm further back to ensure you get it all. If the whole plant is infected then cut back to just above ground level and feed with a liquid fertiliser to encourage new growth.
Mildew: A white powdery fungus. The symptoms and cure for mildew in clematis is the same for all plants that have mildew. Prune well and remove all infected leaves and stems from the plant and the surrounding area to stop the spread, then mulch and water.
Slim flux: This is a rare disease with no cure. The plant has a yellow, orange, white, slimy liquid that oozes near the base of the plant, caused by a chemical reaction. Best practice, dig the plant up, burn it and don't plant a clematis in the same spot.
To summarise:
Group 1: Small flowering species that flower on last years growth and finish flowering by June - Prune JUNE!
Group 2: Large Hybrids that also flower on last years growth and flower May - June - Prune early winter/early spring.
Group 3: Small flowering cultivars, large flowering hybrids and herbaceous species that flower on current years growth - prune back to 25cm above soil level in early spring.