Group 3 – prune to about one foot tall in late winter/early spring (leave a couple sets of big buds)
Group 2 – prune lightly in spring (just a trim), then after the spring flowering show is over, trim back by about a quarter for good reblooming.
Group 1 – prune after flowering if it’s getting overgrown
I started with Group 3 and went backwards because most of the clematis that you will find in the big home and garden stores are going to be Group 3 or Group 2. Each group has it's own general flowering time in addition to the pruning time.
Do I really have to prune my clematis? Nope, you can do whatever you want. One Fantastic Gardener I know believes in never pruning and letting her clematis do whatever they want, and they still flower quite beautifully:
Why should I prune then? Pruning will encourage better and fuller flowering. For some vines, it also enables reblooming. Left unpruned most of the clematis flowers will stay at the top of the plant, and they will be sparser. Pruning, much like pinching your plants, will enable them to become more bushy and full. Pruning is especially important to flowering for Group 3 clematis, as it mainly blooms on new growth. If you don't prune those, they'll eventually start looking pretty leggy and ragged.
What are these letters on my clematis tags? Groups 1, 2, and 3 are also known as Groups A, B, and C, respectively.
How will I remember it all? I make it easy on myself and pretty much buy only Group 3 clematis, with some exceptions. Then I can just go through my garden and prune almost all of them at the same time and the same way. So pick the group that is going to bloom when you want and stick with plants in just that group for one part of the garden. For several online nurseries, you can search by clematis group or type, making it easier to get the type you want.
What if I mess up and prune the wrong way? You might miss out on a season of flowers, but the plant will likely be fine and rebound.
These mainly bloom on new growth. They are known as the Large, Late-flowering group. They usually bloom in summer or early fall. Prune low in late winter or early spring right above a set of buds to encourage a fuller plant. However, if you want the clematis to get more height and flower further up (for example, if you wanted all the flowers at the top of an arbor or to get to the top of a high porch), you can prune further up. Where you prune is where it is going to start branching.
*Some popular Group 3 clematis: Betty Corning (fragrant!), Comtesse de Bouchaud, Duchess of Albany, Jackmanii, Perle d’Azur, Polish Spirit, VenosaViolacea
These generally bloom in the spring on old growth, and then they will rebloom somewhat in the fall on new growth. Thus you should just give it a light trim if you want in the late winter/early spring to get rid of dead wood and trip the tips of each vine to the topmost bud. After the big spring flush of flowers, in order to get a bigger rebloom, trim the plant back by ¼ or even ½. Some varieties just keep on reblooming throughout the summer. Often the double blooming clematis varieties will rebloom with a single type flower.
If you want just one big flush of flowers in the summer, however, feel free to prune Group 2 clematis just like Group 3.
*Some popular Group 2 clematis: Arctic Queen, Dr. Ruppel, General Sikorski, Henryi, Miss Bateman, Nelly Moser, The President
Clematis 'Guernsey Cream', group 2
GROUP 1: These bloom in spring on old growth. You don’t have to prune them, but if they are getting out of control or in order to remove any deadwood, you can prune them right after they flower. These are the earliest clematis to bloom. This group includes the Montana varieties, which can be massive plants full of small very fragrant flowers in the spring. These varieties might be a little harder to find.
*Some popular Group 1 clematis: Alpina Stolwijk Gold, Armandii (evergreen!), Macropetala Lagoon, Macropetala Markham’s Pink, Montana Grandiflora, Montana Mayleen
GROUP 4: Group 4?!! There’s a group 4?! A lot of people group these with Group 3, since the pruning is the same. This is the herbaceous group – they are shorter and look more like a regular plant or a scrambler than a vine. They are a very hardy group.
*Some popular Group 4 clematis: Alionushka, Arabella, Durandii, Integrifolia, Recta Purpurea, PetitFaucon, Virginiana
Prairie Clematis 'Gazelle', group 4
For more explanation on pruning (and for one of the best online clematis nurseries), see Silver Star Vinery.
For some good pruning visual aids, see Completely Clematis Specialty Nursery.