How to Grow Clematis
Clematis is treasured for its marvelous, long-blooming flowers and its manageable growth habit. As a bonus, pretty plumed seed heads that are decorative all by themselves appear after the flowers. There is a clematis plant that is suited to just about any situation and climate. Clematis offers the longest flowering period of any climber, and there are species or hybrids that bloom in nearly every month of the year. Their diverse forms range from herbaceous plants to rambling shrubs, in addition to the better-known climbers. Clematis also demonstrates an enormous range of flower colour and flower shape.
Vigorous clematis that flower in the spring are perfect for decorating and covering up unattractive sheds, fences, and walls, or for scampering over aged trees, and stumps. Anything you want camouflaged is a good candidate for the clematis treatment. Those that are less unbridled can be trained onto trellises and pergolas, or permitted to spill across decks or balconies. On the ground, the more tender clematis will drape where they will be most admired.
Some types of clematis can be grown in containers on patios, or, for those that are more fragile, in a greenhouse. Others can be grown through host plants such as trees and sturdy shrubs – providing an opportunity for interesting, unusual, or unexpected garden highlights.
There are many different species of clematis, along with their hybrids, in cultivation. Most of these are hardy to RHS H7 (USDA zones 4 or 5). In general, they are divided into three main groups. Group 1 is comprised of the early-flowering species and their cultivars, as well as the Alpina, Macropetala, and Montana subdivisions, which flower directly from the previous season’s mature stems. Group 2 is comprised of the early, large-flowered cultivars that bloom on the current season’s short stems, which have grown from the previous season’s longer, mature stems. Groups 1 and 2 are sometimes referred to as old-flowering clematis, which means that they bloom on old wood (the previous year’s growth). Examples of Group 2 include ‘The President’ and ‘Nelly Moser’. Group 3 includes the late-flowering species, late, large-flowered cultivars, and herbaceous (short, more sprawling) types, all of which flower on the current season’s growth. Examples of Group 3 include ‘Jackmanii’ and ‘Mme. Julia Correvon’.
Early, Small-Flowered, Evergreen
The early-flowering evergreen species and their types are mainly from warm climates and therefore must be grown with protection in areas where it gets really cold. The Alpina and Macropetala subdivisions can handle low winter temperatures and are perfect for growing through shrubs and trees that have been trained to walls anywhere. However it is a bad idea to grow them through climbing roses or other shrubs that require annual pruning, since these types of clematis require little or no cutting back. They are appropriate in exposed areas such as the northeast corners of buildings, but also make excellent container plants that can be trained to any kind of support. The members of the Montana subdivision are hardy and vigorous, reaching 22-40ft (7-12m). These types of clematis will cover walls and arbours and are striking when grown through conifers or old fruit trees that are past their prime, but it is possible for them to damage the foliage of evergreens with their uncontrolled, thick growth.
Related: When to Plant Agapanthus
These types of clematis are largely hardy, and grow to a height of 8-12ft (2.5-4m). The more compact cultivars, which are usually the first to flower, are excellent for growing in containers. It’s best to grow the double and semi double cultivars, as well as the midsummer-flowering kinds with really large flowers, through the branches of other wall-trained trees or shrubs, where their flowers will be shielded from severe wind or heavy rain. Cultivars with pale, pink, or mauve-striped flowers do best when planted in the shade, where their flowers can be used to brighten dark spaces and where they will not get sun-bleached. Those with purple or deep red flowers are more suited to sunny places, because they develop better colour in the light and warmth.
This group of clematis includes the late, large-flowered cultivars, late, small-flowered cultivars, and a wide collection of species and their forms, including the herbaceous clematis. These include cultivars that are good for growing through climbing roses, shrub roses, and most medium evergreen and deciduous shrubs, as well as cultivars that are terrific for growing through ground cover plants such as heather. Some cultivars in this group make excellent ground covers themselves. The herbaceous cultivars grow very will with bush roses that have flowers in clusters.
Planting and Soil Preparation
It is safest to buy two-year-old plants that have been grown on their own roots, not grafted. When using vigorous types of clematis to dress up trellises or pergolas, make sure that the supporting structure is strong and sturdy before planting. Likewise, when growing clematis through old trees, make sure the branches are strong enough to bear the weight of a heavy vine. This idea also holds true for clematis grown through other host plants, where it is crucial to match the vigour of the clematis with the vigour of the host, otherwise, the host will be overwhelmed. It’s also important to make sure that the pruning requirements of the host are compatible with those of the clematis.
Most climbing clematis can be placed in part shade or in sun, provided that their roots are cool and shaded. This can be accomplished either by under planting them with low shrubs, placing them on the shaded side of a host tree or wall, or putting something like flagstones or gravel on top of the soil around them. The herbaceous species flourish in sunny locations. To grow clematis on a balcony, the best bet is a container that is 18in diameter x 18in deep (45 x 45cm).
Most clematis will do well in any rich, fertile, and well-drained soil, particularly if it is neutral or slightly alkaline. However, it is important to note that evergreen clematis plants should not be grown in soil that stays wet during the winter months, because they have fine, fibrous root systems that will swiftly rot.
Regular Care and Potential Clematis Problems
Keep newly planted clematis well watered until they’re established, and mulch climbing and herbaceous plants with something non-acid like compost or well-rotted manure annually in the spring.
Fresh young growth is highly susceptible to slug damage in the early spring. Clematis might be prone to attack from aphids, and the later-flowering, large-flowered cultivars might be stricken with powdery mildew. Clematis wilt, a condition that is caused by a fungus, can attack newly planted, large-flowered clematis.
How to Prune Clematis
Clematis pruning requirements vary for each group, so it’s vital to know exactly what kind of plant you’re dealing with before you do any cutting. Otherwise, you run the risk of pruning out the flowering wood altogether. Clematis climb using tendrils on their leafstalks that attach themselves to their support. They need to be trained to make it to their support, and any new growth will need to be tied in during spring and summer. Do this by tying just below a node (or leaf axil bud), and spacing stems evenly on the support, making sure to leave enough room for further growth to develop. Clematis require special care during the initial training and pruning period. If they are left untrained, the plants will often produce one or two stems that are 50-60ft (15-18m) long, followed by a top-heavy morass of growth, leaving the base of the plant bare. To avoid this problem, all newly planted clematis need to be pruned hard the first spring after planting. Unless the plant already has three or four stems growing from the base, cut back all the stems just above a strong, healthy pair of leaf buds about 12in (30cm) above soil level. If this hard pruning does not generate three or four extra stems in early spring, any new stems should be pruned to about 6in (15cm), just above a robust pair of leaf buds.
Many of this group of clematis need little, if any, regular pruning, particularly if they’re cut back hard when first planted. If it’s necessary to restrict the size or tidy tangled growth, pruning should be done after flowering. Any dead, weak, or damaged stems can also be removed at this time. New growth will mature during late summer and fall, and produce flowers the following spring. Any excess growth can be cut during the fall, and tied into its support, but keep in mind that this will reduce the following spring’s flowering display.
The solitary, large flowers are produced on the current season’s stems varying from 6in to 24in (15-60cm) in length, which originate from the previous season’s mature stems. Prune these types of clematis in early spring, just before the new growth begins. Remove any dead, weak, or damaged stems, and then cut back healthy stems to just above a strong pair of leaf buds. These buds will produce the first yield of flowers. This bunch may also produce a second, but smaller, flush of blooms on new shoots in late summer.
The flowers created by this group are on stems of the current season’s growth. Prune in early spring, before new growth begins. Remove all of the stems from the previous season down to a strong pair of leaf buds right above the base of the previous season’s growth, 6-12in (15-30cm) above the soil level. As new growth appears, tie it to its support structure or host plant.
Be careful with the soft, and frequently brittle, new stems. Space them evenly and tie them in at regular intervals. Clematis that grow over host or groundcover plants, such as heathers, should also be pruned. In late fall, remove superfluous growth to keep the plant and host tidy and less susceptible to wind damage. When clematis are grown over winter flowering heathers, all clematis top growth should be removed down to 12in (30cm) in late fall, before the heathers begin to flower.
It is possible to propagate some clematis species by seed sown in fall and overwintered in a cold greenhouse or frame. However, it is important to note that cultivars do not grow true from seed. Cultivars are usually propagated by either softwood or semi-ripe cuttings or via layering; division or basal stem cuttings are the propagation method for herbaceous clematis.