Controlling Clematis Size
I need to put my clematis on a larger trellis. The one I have now is too small to support the plant. I live in zone 5 and don't know quite how to go about doing it.
Try pruning your clematis to reduce the size and eliminate the need for a larger trellis. Since most clematis in northern areas bloom in summer or fall on new growth you can cut the plant back to 12 inches in late winter. This will encourage more growth at the base of the plant and reduce the overall size. If a bigger trellis is needed I would tackle the job in spring after pruning the plants. Plan ahead so you can remove the old and install the new trellis with minimal disturbance to the plant's roots. If space is limited, you may want to retrofit the old trellis to accommodate the larger size. Fortunately plants are tougher than we think. Increase your chance of success and reduce stress caused by changing the trellis with proper care throughout the growing season. Mulch the soil around the clematis and water thoroughly whenever the top few inches of soil are crumbly and slightly moist.