When it comes to spacing plants, the short answer is to take the Goldilocks approach. Too close and the plants will out compete with each other, resulting in dead or thinning plants and a messy composition. Too far apart, and the plants will never meet and weeds will grow in the gaps. To get the spacing just right, work out how large the plants will grow and then space them so the branches will eventually slightly overlap. For example, plants that grow 1m wide should be spaced approximately 800 - 900mm apart. However there are always exceptions to the rule - sometimes the Goldilocks approach leads to a boring and predictable composition! It’s ok to break the rules as long as it is done intentionally!
So, how do you intentionally adjust plant spacing to get interesting results, and when is it ok to over or under plant?
Life span.
Some plants grow slowly and are long lived, others grow a little faster and fade out within a few years. When using the Goldilocks approach it is a good idea to fill the short term gaps with short lived species. By intentionally placing longterm plants where they can slowly fill the space, you design in longevity and maintaining the garden becomes easier over time. Interplanting with short lived plants provides the instant garden look and helps to suppress weeds. Care should be taken to make sure the fast growing plants never swamp the mainstay plants. When it is time to remove short lived plants, small gaps will open up in the composition. You must resist the urge to replant, and have faith in your spacing, allowing the long lived plants to do their thing.
Lavender is a wonderful filler plant - it lives fast and dies young, plus it is easily clipped to keep to it from swamping the mainstay plants.
Interplanting and underplanting.
Plants with different habits and growth cycles can be planted very close together as long as they can co-exist. Forests are a good example of interplanting. Each layer of the forest occupies a different vertical space. Ground covers and grasses grow under the canopy of large shrubs; the shrubs grow under the small trees and the tall trees grow above everything else. It is possible to plant things really close together as long as they can co-exist vertically.
It is possible to copy the vertical layers of a forest and pack plants in.
Meadows and perennial boarders are also a good example of both interplanting and underplanting. The plants in a meadow all have different habits, and some perennials and grasses are tall and thin and can easily co-exist with short and fleshy plants. Many of the plants in meadows die down at a specific time of year. This means that they can be underplanted with plants that have the opposite growth cycle. A skilled gardener can design the underplanting of bulbs and perennials to provide year round interest.
Meadows have plants that grow and flower at different times of the year, this helps the plants to co-exist in very close spacing.
Intentional elongation.
If you plant trees too close together, the lower branches will thin out. The trees will also take on a stretched or elongated look as they reach toward the Sun. If you use the Goldilocks approach when spacing trees, they will grow in a text book way and take on the shape shown on the ‘label’. However, to encourage an etherial and graceful habit, most trees are more then happy to be planted very close together. When planting trees close together it is important to consider any footings and paths. Another consideration is that the variety of understory plants that are happy to grow amongst a mass of tree roots is very limited.
Planting trees close can create an etherial look
Happy mistakes.
Spacing mistakes are usually caused through eager gardeners wanting an instant result or thrifty gardens wanting to save some money. In both cases, the composition usually fails, however sometimes incorrect spacing can lead to a happy mistake. The best example of a happy mistake is an evergreen hedge where the plants are spaced too close together. To maintain full foliage cover from top to bottom all evergreen hedge plants, including Conifers, Lilipilly and Ficus, need to follow the Goldilocks approach. Too far apart and you get spongy sections that are difficult to clip. However, if you plant too close, the lower branches will naturally thin out, effectively pleaching the hedge. A pleached hedge is where the lower branches are removed, creating a feature of the repeated trunks. If pleaching is the look you are after that is great, however if you really want to hide the neighbours, then spacing the plants too close is the last thing you want to do.