That plant tag photo can be very persuasive. It depicts a shrub or perennial or groundcover that appears to be the answer to that empty garden bed. The flowers are colorful. The plant is full. It looks nice and tidy in its small pot. But this photo is merely a hint in your landscape design drawing. You also need to consider mature height, mature spread, spacing, sunlight exposure, water requirements, and the location of the plant relative to paths, borders, windows, and seating.
Novice gardeners may design based on the size of the plant as currently sold. A small, tight shrub that is perfectly sized to stand next to a pathway today may spread too far into that pathway when it matures. A young perennial that appears too short in the pot may look right when the plant has matured to its full spread. Placing plants as if they are the size of the pot when they will soon be much larger can result in a crowded first year of the landscape design.
Before you assign names to a planting design, you can place circles on your base map to show the mature spread of each plant. You do not have to make these circles pretty or precise enough for engineering work. They just need to remind you that the plant will fill that circle’s area. In order to practice this, use a sketch of an existing landscape design as a test. On the graph paper or tracing paper that you use for planning, trace or lightly sketch the existing garden bed edge, pathway, lawn edge, fence, patio, or tree, then use circles to represent the mature spread of each plant that you are contemplating. Allow plenty of space for mature spread between those circles. You might see that your border is too crowded, or a pathway is too narrow.
Mature height is equally as important as mature width. A tall shrub or groundcover that is planted in the front of a bed will obscure plants that are located further back. An extremely low plant might look too low and invisible if placed too far back from an important vantage point. Mature height will impact what the plant will look like from windows and seating areas. A group of plants that looks just right from above might look obstructed, flat, or unbalanced when you look out at it from a window. Layer your planting design so that low plants are closest to the pathway, mid-height plants are located more in the middle, and taller plants are in the rear where they will add privacy, structure, or an unobtrusive background.
The photo can help you determine if the plant color appeals to you, but it is the least important consideration when deciding which plants to select. A beautiful color may only last a short period of time. Leaf texture, mature shrub layer, mature spacing, and maintenance needs are more significant. A plant with great blooms may need to be pruned more easily than the space it will fill. A plant that requires full sun may be in the partial shade of the area where you selected it. A plant may grow faster and larger than the space it will fill and may cover an edging, mulch layer, or small perennial. These considerations will help make the landscape easier to maintain, whereas the photo will make the space look harder to maintain long-term.
Try a planting design exercise without purchasing anything. Select a garden bed, border, or patio edge, then consider what function you’d like each plant to perform (provide a height, provide contrast in texture, provide interest during the season that it is not at its peak, and fill the ground). Then look at each individual plant’s mature size and spread to ensure that they perform that function. If two plants are fighting for the same spot, or if one is going to be taller than the main viewing area, then remove them from the design before you have them growing in your garden.
Your design needs to account for mature space as well as current space. Those open spaces on the sketch that you are trying to fill may represent mature root area, branches, pathways, the mulch layer, watering access, and plant growth. When you select a plant, let the plant photo appeal to you, then look for plants that will mature appropriately in your design. Is the question, Does this plant look great? Or Is the question, Will this plant fit in the design as it grows to full size?