A trellis is a garden structure that supports plants that may be trained to grow up its sides. To design a trellis you must decide where it's going to be located and what you will use it for.
Where Will it Be Situated?
You will want to protect your trellis from any weather to which it may be exposed. If you're designing an outdoor trellis that will stand in soil, you will have to weatherproof it. If, on the other hand, it's going to be placed under a covered patio on tile or concrete, moisture will not be such a destructive force.
What Purpose Will it Serve?
Trellises intended to support rambling plants need to have enough horizontal and vertical supports to allow the vines or branches to weave through. If your trellis isn't going to support plants, it can be designed to make an independent architectural statement.
What Materials Should You Use?
Decorative patio trellises that are not likely to see more than a nominal exposure to plants should be made with materials that enhance their appearance. There are exotic hardwoods that can be used to create an impressive frame which can then be embellished with such materials as bamboo or raffia.
Outdoor trellises that will be used to support vegetation need to have resilience in the materials from which they are constructed. A trellis destined to be swathed in rambling roses, a grape vine, or ivy does not need to be more than attractively functional and the materials used, utilitarian.
Does Size Matter?
Sometimes a trellis design can be made to use smaller component trellises rather than one big one. If you want to cover a 30 x 10-foot wall with climbing plants it is not necessary to have a trellis the same size as the wall. Imagine how much more effective and attractive five 3 x 10-foot trellises would be if spaced along the wall. It is not difficult to encourage climbing plants to travel between trellises.
Freestanding or Attached?
Free-standing trellises are rare and tend to be either ornamental or designed to hold small pots of plants at different positions. They are also more often seen indoors. If your trellis will not be supported by a building or a wall, your design should include legs that can be sunk two or three feet into the ground.
Traditional or Contemporary?
While much of life changes faster than we can keep up with, gardens seem to have changed hardly at all. If you are designing your trellis to go into an established garden, it is important to be aware of the feel of the garden. If it feels traditional, as though it has been there forever, design your garden trellis accordingly. Your trellis should accent your garden in a striking, but not jarring, way.
A trellis can be a simple structure that will eventually disappear under lush plantings or it can be an architecturally interesting focal point. Once you begin to consider the possibilities, your design can take almost any shape.