The summersweet I planted now stands a full 5 to 6 feet (1.5 to 1.8 m) tall, a Ruby Spice that has taken over its damp corner in Connecticut. It went in as a sad one-gallon pot with maybe three thin stems. Each season after that it pushed up new growth, never in a rush, but it never stalled either. The summersweet growth rate is moderate, a steady medium pace that fills a space over a few years rather than all at once.
That corner stayed wet most of the year, which is the kind of spot this shrub loves. The first spring it barely doubled in size. The second year it added a clear foot of new wood and started to look like a real shrub. By the third summer it had genuine heft to it, with stems thick enough to hold up the flower spikes. By the fifth it was the plant you see now, blooming pink and pulling in bees from the whole yard. No fertilizer, no fuss, just a wet spot and a few seasons of patience.
A moderate pace works out to about 1 to 2 feet (30 to 60 cm) of new growth a year once the plant settles in. The number swings with the soil under it. Rich, damp ground pushes your shrub toward the top of that range, and you get fast fill in a season or two. Dry, lean soil drops it to the low end, and the plant takes its time. Water is the lever here. Keep the roots moist through summer and your summersweet rewards you with strong yearly gains. Let it dry out and growth slows to a crawl, which is the most common reason people think their plant is stuck.
The first year after planting almost always looks slow, and that part is normal. Your shrub is busy underground, building roots before it spends energy on top growth. You might see only a few inches of new stem that first season. Do not read that as a failure or rush to feed it. The real push comes in years two and three once those roots are set, and that is when the moderate summersweet growth rate starts to show.
The summersweet mature size for the common species runs 5 to 8 feet (1.5 to 2.4 m) tall and 4 to 6 feet (1.2 to 1.8 m) wide. That is a sizable shrub, wider than many people expect when they buy that little starter pot. So how big does Clethra get if you want something smaller? Dwarf cultivars are bred to stay compact, holding at 2 to 4 feet (0.6 to 1.2 m) in both height and spread.
Pick the cultivar that matches your space from the start. Do not buy a full-size summersweet for a tight bed and plan to keep it small with shears. Fighting a shrub that wants to be 8 feet (2.4 m) every year is a losing game, and it wrecks the loose, natural shape that makes the plant so pretty. A dwarf form in a small spot saves you that work and looks better doing it. Read the tag before you buy, because the species and the dwarf forms often sit side by side at the nursery in pots that look the same size.
Give your shrub room to fill its full mature size when you plant it. Space it 4 to 6 feet (1.2 to 1.8 m) from the next plant if you went with the species, or about 3 feet (0.9 m) for a dwarf. That looks like too much gap the first year, when the plant is small and the bed looks bare. By year three you will be glad you left the space, since a crowded summersweet grows leggy and thin as it reaches for light.
Plan for several seasons before your shrub hits full size, even with steady moisture. Three to five years is a fair window for a healthy plant in good soil. The wait is short for a shrub that lives for decades and blooms harder each year. Give it the wet feet it wants, mulch the base to hold moisture, leave the pruning shears in the shed, and let the moderate pace do its work.
Read the full article: Summersweet Shrub: Care and Growing Guide
