Determining the number of clematis plants needed for a trellis requires assessing several variables. These factors influence the plant’s mature size and the support structure’s capacity. Understanding the clematis variety’s growth habit and the physical dimensions of the structure ensures the vine reaches its full flowering potential while maintaining long-term health.
Understanding Clematis Growth Vigor
The most significant factor determining plant spacing is the clematis variety’s inherent growth vigor, which is categorized by its pruning group. Clematis are sorted into three groups (Type 1, 2, or 3) based on when they bloom and what wood they flower on. This grouping provides an immediate indication of how rapidly and extensively the vine will grow once established.
Type 3 clematis, such as the Viticella and Jackmanii varieties, are the most vigorous climbers because they produce flowers exclusively on new growth each season. Since these are pruned back hard annually, they generate a significant amount of new stem length, often reaching 10 to 12 feet in a single growing season. This rapid growth necessitates the greatest distance between individual plants on a trellis.
Type 2 clematis are large-flowered hybrids that exhibit a more moderate growth rate compared to Type 3, often blooming on both old and new wood. These plants typically require less space to reach their mature spread. Type 1 clematis, which include species like Clematis montana, are also extremely vigorous and often require minimal pruning, making them better suited for covering large fences or buildings rather than standard trellises.
Assessing Trellis Size and Support
The capacity of the support structure is the second major variable in the plant count equation. Before planting, a gardener must measure the trellis’s total surface area by multiplying its height by its width to determine the available square footage. A standard trellis might be 6 feet wide by 8 feet high, offering 48 square feet of climbing space.
The physical structure of the trellis also dictates the maximum plant material it can safely hold. A narrow obelisk or a lightweight plastic lattice will not support the same weight as a sturdy, wall-mounted wooden frame or a heavy-duty arbor. Selecting a highly vigorous variety for a lightweight structure can result in insufficient support for the mature vine.
Trellises can be broadly classified by size, which helps standardize the planting approach. Small trellises, those under 25 square feet, might only accommodate one moderately vigorous vine. Large trellises, exceeding 50 square feet, can potentially host multiple plants. The structure’s strength must match the combined mature weight and bulk of the chosen clematis variety.
Calculating Plant Count for Desired Density
Combining clematis vigor with trellis size allows for precise plant count calculation. Spacing recommendations ensure each mature plant has sufficient room without competing excessively for light and nutrients. Vigorous Type 3 clematis, like Clematis viticella varieties, require a minimum spacing of six to eight feet to prevent dense, tangled growth that restricts air circulation.
For less vigorous Type 2 hybrids, such as ‘Nelly Moser,’ a tighter spacing of three to five feet is appropriate for full coverage. To calculate the number of plants, divide the total trellis width by the recommended spacing distance. For instance, an 8-foot-wide trellis supports only one Type 3 clematis planted centrally, as the width falls within its mature spread.
If that 8-foot trellis uses a Type 2 clematis requiring four feet of space, two plants are needed, spaced four feet apart. This strategic placement prevents the canopy from becoming overly dense, which restricts airflow and increases fungal disease risk. Planting for long-term health means respecting the vine’s mature width, even if full coverage takes longer.
Choosing closer spacing for immediate coverage requires dedicated pruning to manage competition. Overplanting often causes lower stems to become bare, restricting flowering to the top of the trellis where light is available. Adhering to established spacing ensures the vine remains healthy, with blooms distributed evenly from the bottom to the top.
Optimal Planting and Training Techniques
Once the correct number of plants is determined, the planting method is instrumental in establishing a strong, multi-stemmed vine. Clematis, especially Type 2 and Type 3 cultivars, benefit from being planted deeper than in their nursery pots. Position the top of the root ball two to three inches below the final soil level to encourage new shoots from below the ground.
When planting near a solid structure, dig the hole at least 18 inches away from the base so roots can access sufficient moisture. Angle the plant toward the trellis, using a temporary cane or wire to guide initial growth until stems reach the permanent support. This angling prevents the roots from being shaded and dried out by the overhead structure.
Immediately after planting, begin training the first new stems by gently tying them horizontally or vertically to the trellis with soft ties. Clematis climb by wrapping their leaf petioles around thin supports, so providing an initial framework ensures efficient coverage. The roots prefer to be cool and shaded, which is accomplished by planting low-growing, non-aggressive annuals or perennials at the base of the vine.