Many people regard the pruning of clematis as a complicated area, but in fact as long as the gardener understands the flowering habit of clematis it is perfectly simple. Clematis are basically old- or new-wood flowering.
The confusion about clematis pruning that has arisen was engendered by the original breeding carried out in the 1850s and 1860s. The breeders referred to clematis as evergreen, C. alpina types, C. patens types, C. lanuginosa types, C. jackmanii types, C. viticella types large or small-flowering and, of course, the miscellaneous later flowering species and their cultivars.
Pruning should be carried out with the aim of producing the maximum amount of flowers on the clematis plant. Therefore, once the main flowering period of the clematis species or cultivar is known, the question of pruning becomes very simple. If you prune an old-wood flowering clematis hard, no harm is done; the first main crop of flowers will be lost but these will be replaced by a later batch of flowers which will be smaller but possibly more profusely borne. The growers of the old-wood flowering clematis in Northern Europe and colder areas of North America sometimes have no choice - their clematis top growth is killed right down to soil level by the frosts and winter desiccation. The clematis then regrow each year from ground level, producing flowers in late rather than early summer.
During mild winters, especially in warmer parts of Europe, some clematis may continue to grow slowly, or new growth may appear too early. It is tempting to get underway with pruning early, but this may encourage fresh new growth that will sometimes be killed by severe late spring frosts. The best growth may thus be destroyed, so it is advisable always to wait until the correct pruning time.
Newly planted clematis
All newly planted clematis require hard pruning during the first spring after planting, regardless of which group they belong to, unless they are extremely strong with plenty of stems growing from the base. All top growth should be reduced down to approximately 30-45 cm (12-18 in) so that young growth will be produced from the base of the plant.
Pruning of clematis on groundcover plants
Clematis scrambling over evergreen groundcover plants such as heathers should be cut back by two-thirds in late autumn. With clematis belonging to the Viticella section growing on or through winter-flowering heathers, all top growth should be removed back to within 30 cm (12 in) of ground level at this time.
Pruning to change the flowering period
If a clematis cultivar from pruning group two has been selected for its flower color to blend with, for example, a rose, it may be pruned as for group three clematis. Although the early flowers will be lost, flowers will be produced to coincide with the blooming of the rose. The clematis to select here are the repeat-flowering cultivars from group two such as C. 'Elsa Späth', C. 'General Sikorski' or C. 'Niobe'.
Mid-season large-flowered cultivars
Certain large-flowered cultivars which produce their first flowers in early summer on long stems that have grown from the previous season's ripened leaf axil buds can, if required, be pruned as per group three, rather than group two. This will cause the loss of some early flowers but the plants will benefit, growing vigorously and producing flowers over a long period from early summer to mid-autumn. These clematis, bred from the old C. lanuginosa types, do not quite fit into group two or group three, so with experience they can be pruned either way. In northern Europe or northern North America, these clematis generally lose their top growth in severe winters in any case.
Summer pruning & deadheading
Some clematis growers prefer to remove spent flower heads to encourage further crops of flowers, especially with the early large-flowered single, double and semi double clematis. Certainly, if the old flowers are removed with a length of stem with 2-3 nodes, new growth will appear and a further crop of flowers will be produced. When this is done, it is important to keep the clematis well watered and fed. The only drawback to this is that the attractive seed heads on this group of clematis will be lost. A compromise can be achieved by removing only 50 per cent of the spent flower heads, retaining the remainder to develop into the fluffy flower heads which are also important. This is particularly to be recommended when clematis are cultivated in containers.
Pruning mixed groups of clematis
Many people like to grow clematis belonging to all three pruning groups together to obtain a long flowering period. However, growing clematis which belong to pruning groups two and three together is quite feasible. If the wrong stems are cut or damaged, all is not lost for the following season -it just causes a delay in flowering. Growing the pink, blue and white C. alpina and C. macropetala types together does not cause any problems at pruning time. In containers, planting two or even three cultivars belonging to group two can be very effective and if they have slightly different flowering periods so much the better.
The importance of giving clematis a sufficient amount of water cannot be overestimated; after watering, you can almost see the plant growing. In theory it is possible to overwater; a hose directed continually at a piece of ground would ultimately leech all the nutrients out of it. In the amounts recommended for a clematis, this is unlikely to happen and in any event is counterbalanced by the rich feeding programme.
A clematis plant requires a minimum of 5 liters (1 gallon) of water a week. But it will profit from as much as 20 liters (4 gallons) per plant per week if possible. During very hot weather it may require 5 liters (1 gallon) per plant per day. It is best to 'point' water, i.e. to direct the water specifically on to the clematis plant rather than assume it will take its share from a more general garden watering; in this way you can determine exactly how much water you are giving. Watering should take place out of the sun in the evenings. If a hose is used, then use a fine spray on both sides of the leaves. If a pipe is being used alongside the plant, then put two- thirds of the water into the pipe and the rest over the soil to keep moist any roots which are near the surface. If a liquid fertilizer is being used at the same time, this can be placed in a watering can and the water added from the hose so that the fertilizer is dissolved and diluted; the water with the diluted fertilizer is applied from the can. Watering will, of course, be assisted by having planted your clematis correctly with a saucer area at the top of the hole.
The main reason for using a mulch with clematis is to retain the moisture in the ground. This is much more effective than planting the roots in the shade, or planting shrubs around it. Additional reasons for using a mulch are that it keeps the ground cool, it suppresses weeds, it adds humus to the ground and it will also help to add nutrients to the soil. If sterilized mulch material is used, for example peat /or sterilized mushroom compost, then it may help to protect the clematis against wilt as it will not contain spores of the fungus.
Before applying the mulch, remove any dead material on the ground and burn it. Add any fertilizers that are required and, if the ground is dry, water it. Apply sufficient mulch material to cover 60 sq cm (2 sq ft) around the plant, and to a thickness of at least 5-8 cm (2-3 in) - 8-10cm (3-4 in) would be better. Do not carry the mulch material close to the stems. In the autumn the material can be forked gently into the ground or left to protect the roots against a severe climate.
The following materials can be used:
If you use materials which are liable to take nitrogen out of the soil, then 60-90 g (2-3 oz) of sulphate of ammonia can be spread over the ground to the sq m (sq yd) before applying the mulch. The mulch should be applied in the spring after the soil has warmed up. Later, inorganic fertilizers or liquid fertilizers can be applied through it. Recently there has appeared on the market a porous sheeting which prevents weeds coming through, retains water in the soil and at the same time, being porous, allows water and fertilizers to pass easily through.
Plants can, of course, create shade but they also tend to take water and nutrients out of the ground. The most suitable plants to use for shading are primulas, pansies, dwarf lavenders and the small potentillas.
Clematis are generally hungry feeders and will take whatever is offered. In the open ground, feeding during the growing period and annual mulching must be considered for the continued well-being of the plant; feeding will encourage better growth, more flowers and a much healthier plant. A spring-planted clematis will benefit from additional feeding. This can be carried out at the time of watering by the use of liquid fertilizer. There are many of these available, or any good general feed can be used instead. Those which can be used as a foliage feed can be most beneficial and green up the foliage. A simple liquid feed used for roses is quite sufficient. The amount of feed and the frequency of feeding will depend upon the type used and careful attention should be paid to the manufacturer's recommendations. Resuming after flowering will encourage further growth and, with luck, another crop of flowers. If feeding commences during flowering, the flowering period will be ended prematurely.
An annual mulch in late winter or early spring will help feed the plant in later months and a thick mulch will also give added shade to the root system. A 7.5 cm (3 in) layer of well-rotted farmyard manure, spent hop waste or well-rotted garden compost is ideal. Do not place the mulch close to the base of the stem of the clematis-leave a gap of 10-13 cm (4-5 in) diameter or harmful gasses may cause burning to the new stems and growth that may appear from below soil level. A frosty day when the garden soil is frozen is an ideal time to do this rather than damaging wet winter soil by walking on it. If none of the above materials are available, a mulch of peat or peat substitute mixed with two handfuls of bone meal per bucket of peat will suffice. However, peat should be lightly forked in otherwise it will become dry in early summer and be blown away. Any general garden fertilizer can also be used according to the manufacturer's recommendations.
Clematis growing in containers will need regular feeding. For newly planted clematis, liquid feeding is best. This can be carried out at the time of watering. Again, any general soluble feed can be used - rose feed is ideal. A feed of Phostrogen is also most beneficial for container-grown clematis.
For established plants growing in containers, it is important to renew the topsoil in the container each spring, ideally in the middle of the season. This can be done when any winter- or spring-flowering plants have finished flowering and are to be removed to make room for new plantings. Replace the top 5-7.5 cm (2-3 in) of the soil with a good potting compost to give a fresh start to the clematis and additional feed for the summer bedding plants. When removing the old soil, take care not to damage the clematis plant's root system or that of any other permanent plant in the container.
Almost every fertilizer contains three main ingredients: nitrogen (N), phosphorus or phosphate (P) and potash (K). Nitrogen encourages leafy growth and is the key element, because healthy foliage is essential for an overall healthy plant. Phosphate is especially valuable in encouraging root development and so is very useful at planting time to aid initial establishment. Potash plays an important part in promoting flowering and fruiting, especially significant with a flowering ornamental like clematis. Most blended fertilizers also contain greater or lesser amounts of other, minor nutrients like magnesium, calcium and boron.
One other nutrient, however, is particularly important. Iron helps in the process by which the green coloring matter, chlorophyll, is manufactured but plants find it difficult to take up iron from alkaline soils as it becomes chemically attached to other elements. It's true that, as they are naturally adapted to alkaline soils, clematis can absorb iron better than plants than naturally prefer acidic conditions. It's important, however, to use a fertilizer containing sequestered iron; that is, iron in an organic form that does not become chemically 'locked up' in the soil.