That plant in the pot looks benign. It might be small enough to lift in the palm of your hand, with nicely shaped leaves, compact roots, and easy-to-visualize in the front of a border. But the pot is the plant in the pot, today, not what it will want tomorrow. One subtle detail to remember as you plan a new garden layout is that mature plant size matters more than pot size. In many instances, a tiny shrub in a nursery pot might look like it’s an appropriate size for a narrow planting bed, even when the label indicates a plant with a mature spread that is larger than the planting bed.
A small plant might look quite diminutive in the spring, but by mid-summer be a large patch, while a tiny, delicate-looking ornamental grass in a nursery container might grow to be the dominant plant in a border, pushing its way into the path, seating area or smaller plants at the edge of the bed. It’s important to keep in mind that if your garden design is based solely on what plant size looks like in the pot today, that new design is destined to get crowded before it’s even been planted.
Remember that the plant is not a flat object that will stay in position forever in a border, so when planning your border layout, you should think in terms of future plant size. It might be hard to water, weed, prune or simply walk along that border when the plants have grown to their full size. A shrub can block or narrow a path, and a tall plant can block views to your garden from a window, or a small seating area can start to feel too small if a dense shrubbery is right next to it. So keep mature plant size in mind in order to preserve your future garden both for beauty and use.
Take a look at each plant’s tag or catalog description for mature plant height and spread before you place a plant on your sketch and mark the size on your garden plan with a circle or oval (don’t make your circles or ovals too tight). This simple trick helps you to see the space around a plant and will inform your choices. Will the mature spread of that shrub run into the path? Do your perennials look good with the amount of room they have, so the whole group can repeat as it grows larger? Should that planting bed be designed with fewer, larger varieties to leave room for mature plant size? It doesn’t need to be a precise scale drawing, it just needs to remind you that each plant on paper has a future size as it grows.
Often, beginners worry that plants will look too widely spaced at first. Of course, there may be more space to cover, especially when new plantings have bare soil or mulch. Don’t feel the need to plant too densely. In the meantime, think about what the young garden bed will look like and layer it with ground covers, mulch, edging, repeated low-growing perennial species or similar plants. This will help it look good in the meanwhile, even though you leave gaps for future growth. Don’t be afraid to leave room in a garden design! It’s not a design mistake; it’s actually part of your garden design!
Mature plant size can also contribute to a more relaxed plant palette. Once you have a better understanding of what each plant requires, it’s easier to choose fewer plant types and repeat them, creating a more rhythmic effect. Three well-placed groups of the same perennial plant will look much more unified than ten single plants placed side by side to create a group, so this approach often works really well in a small backyard or front yard garden design where you can appreciate more simple shapes, plant textures and layering and less busy, crowded details.
So before you plant, use mature plant size as one way to review your planting plan and make adjustments. Check your sketch to see if each plant has ample room to spread, and if you’ll have access to that space to water and maintain it. Is the plant still clearly visible when it reaches its mature size? Will paths, borders, seating areas or focal points be compromised? If you are unsure, try removing one plant from your design, or plant them with more space or place the biggest plant in a better position. Planting a garden is more fun if you design the space as the plants want to be, not just as they look in their nursery pot.