When planted right and in the right spot, Clethra, also called summersweet or sweet pepperbush, are very easy to grow. In landscape design, they are especially effective massed or grouped in sunny or shady flowering shrub borders, butterfly and hummingbird gardens. Clethra are one of a few summer-flowering shrubs that will bloom in heavy shade so are an excellent choice for woodland gardens. Also suitable for moist soils along stream banks and around ponds. Plant near a patio or other outdoor living spaces to enjoy the fragrant summer flowers. 

Here's a breakdown of what you need to know...


Cultural Preferences



Soil Preferences

Clethra are easily grown in a wide range of soils types, including clay soils. They prefer a consistently moist to wet acidic soil. It's best not to allow the soil to dry out completely. Give plants an occasional deep soaking during drought.


How To Test Soil Drainage  

If you are uncertain about soil drainage in the area you intend to plant your Clethra, it's well worth taking the time to test the drainage before planting. To test soil drainage, dig a hole 12" wide by 12" deep in the planting area. Fill the hole with water and let it drain. Then, after it drains, fill it with water again, but this time clock how long it takes to drain. In well-drained soil the water level will go down at a rate of about 1 inch an hour. A faster rate, such as in loose, sandy soil, may signal potentially dry site conditions and a need to add some moisture retentive organic matter. A slower rate indicates poor draining soil, which Clethra has no problems with!


Soil pH

Clethra grow best in an acid to neutral soil ranging from around 5.0 to 7.0 on the pH scale. Most average garden soils fall between a pH range of 6.0 to 7.0.

Testing Soil pH 

Soil pH is a measurement of the alkalinity or acidity of soil and is measured on a scale of 1-14, with 7 as the neutral mark. Any measurement below 7 indicates acid soil conditions, and anything above 7 indicates alkaline. 

If you're unsure about the pH of your soil, and whether or not it's suitable for growing Clethra, it's a good idea to test the soil pH in the planting area. You can quickly test soil pH with an inexpensive soil pH tester probe. To raise the pH (make more alkaline) you can add pelletized limestone to the soil. To lower the pH (make more acid) you can apply Soil SulfurAluminum Sulfate, or Chelated Iron. Adding organic compost to the soil or using compost as mulch can also help to increase acidity and maintain acid soil conditions.




Light Preferences

Clethra are one of a few flowering shrubs that will grow and flower equally as well in sun or shade.




Planting Clethra (Summersweet)

Scroll down for container planting instructions and care tips



Step 1

Start by digging your planting hole at least two to three times as wide and as deep or not much deeper than the root ball of your plant. The wider the hole the better. Place native soil removed from planting hole around the perimeter of the hole, in a wheel barrow, or on a tarp.


Step 2

Depending on the type, fertility and porosity of the soil in the planting area, it may be beneficial to amend the native soil. When planting in heavy clay soil mix in a good soil conditioner, planting mix, or composted organic matter at a 50/50 ratio with the soil removed from the planting hole. When planting in a sandy, quick-draining soil amending with top soil, organic compost, or peat moss will help to retain moisture and supply vital plant nutrients. When planting in fertile, loamy, consistently moist soil there may not be a need to amend soil.


Step 3

To remove your Clethra from the container it was growing in, squeeze the sides of the container and then grasp the base of the plant with your fingers and try to gently lift and remove it from its container. If the root ball is stuck in the container either cut the container use snips to cut the container away. After having removed the plant from the container, carefully loosen some feeder roots around the surface of the root ball. If your plant is root bound, you can spray the sides and bottom of the root ball with a stream of water from a garden hose. This will help to wash away some soil from the exterior of the root ball making it easier to loosen roots.


Step 4

Set your Clethra in the planting hole so that the top edge of the rootball is at ground level. It may be necessary to place some of your backfill soil mixture in the bottom of the hole to achieve proper planting height.



Step 5

After setting your Clethra in the planting hole, use one hand to hold the plant straight while using your other hand to begin backfilling the soil mixture around the root ball, tamping as you go to remove air pockets. When you have filled the hole to the halfway point you can soak the soil. Then continue backfilling to the top edge of the root ball. To avoid suffocating your plant, avoid placing any soil on top of the root ball.


Step 6 (Optional)

If you are planting your Clethra in a location that is far from a water source, you can use remaining soil mixture to build a water retaining berm (catch basin) around the outside perimeter of the planting hole, as shown in the illustration above. This basin will help to collect water from rainfall and irrigation often reducing the need hand-watering. The berm can be removed after a year or so or when the plant has established itself.


Step 7 

Next, deeply water the planting area, including the root ball, to a depth equal to the height of the root ball. For an extra boost, to stimulate early root formation and stronger root development, you can also water you newly planted Clethra with a solution of Root Stimulator, which reduces transplant shock and promotes greener, more vigorous plants.


Step 8

Apply a 1 to 2" layer of cured, shredded or chipped wood mulch or pine straw around the planting area to conserve moisture and to suppress weed growth. As the mulch decomposes it will add vital nutrients to the soil that your plant will appreciate. Avoid the use of freshly chipped or shredded wood for mulch until it has cured in a pile for at least 6 months, a year is better. Avoid placing or piling mulch directly against the base of your plant as this could cause the bark to rot.



How To Plant A Clethra In A Container


Clethra growing in pots require a consistently moist soil. I recommend using a quality potting soil or potting mix, or a 50/50 combination thereof.  

Make sure to choose a container with drainage holes at the bottom and one that is large enough to allow for 2 to 3 years of growth before shifting up to a larger size container. This might mean your planting pot would be 8 inches or more in diameter (width) than the root ball of your plant. 

Container color will matter as well. Not only will you want to pick a color of container that goes well with the flower and foliage color of your Clethra, you'll also want to pick a container that matches the style of your home or other structures and other plants in the surrounding environment. 

Many nursery & garden centers offer a wide variety of containers to choose from. Before heading out to buy a container take pictures of your home and the surrounding environment. Doing so will help you to choose just the right color and style.



Container Planting Instructions


Step 1

Before filling your container with the soil mix, I recommend lining the bottom with shade cloth or a porous landscape fabric. This will keep the drain holes from becoming stopped up with soil. 


Step2

To remove your Clethra from the container it was growing in, squeeze the sides of the container and then grasp the base of the plant with your fingers and try to gently lift and remove it from its container. If the root ball is stuck in the container either cut the container use snips to cut the container away. After having removed the plant from the container, carefully loosen some feeder roots around the surface of the root ball. 


Step 3

Pour a small amount of your soil mixture in the bottom of the container. Set the Clethra in the container and make necessary adjustments by adding or removing some soil so that the top edge of the root ball will sit 1 to 2 inches below the rim of the container.



Step 4

Backfill with your potting soil around root ball, tamping as you go, until the level of potting soil is even with the top edge of root ball.


Step 5

Water thoroughly until water starts to drain from the holes in the bottom of the container. Add more potting mix if settling occurs during watering.


Step 6 (Optional)

Apply a 1/2" layer of wood chips or sphagnum moss to soil surface to help conserve moisture. 





How To Care For Clethra (Summersweet)

Feeding - Watering - Pruning


How To Fertilize Clethra (Summersweet)

Clethra are light feeders however will benefit from fertilization. Feed Clethra in early spring with a slow-release shrub & tree type fertilizer or a natural, organic plant food.  


Soil pH 

Clethra grow best in an acid to neutral soil ranging from around 5.5 to 7.5 on the pH scale. Most average garden soils fall between a pH range of 6.0 to 7.0.


Testing Soil pH 

Soil pH is a measurement of the alkalinity or acidity of soil and is measured on a scale of 1-14, with 7 as the neutral mark. Any measurement below 7 indicates acid soil conditions, and anything above 7 indicates alkaline.

If you're unsure about the pH of your soil, and whether or not it's suitable for growing Clethra, it's a good idea to test the soil pH in the planting area. You can quickly test soil pH with an inexpensive soil pH tester probe. To raise the pH (make more alkaline) you can add pelletized limestone to the soil. To lower the pH (make more acid) you can apply Soil SulfurAluminum Sulfate, or Chelated Iron. Adding organic compost to the soil or using compost as mulch can also help to increase acidity and maintain acid soil conditions.





How To Water A Clethra (Summersweet)

Clethra prefer a consistently moist to wet soil so it's best to plant them where the soil tends to stay moist. 


At Planting Time 

Immediately after planting deep soak the soil in the planting area, including the rootball, to a depth equal to the height of the root ball. For an extra boost, to stimulate early root formation and stronger root development, you can also water you newly planted Clethra with a solution of Root Stimulator, which reduces transplant shock and promotes greener, more vigorous plants.


Thereafter

During periods of drought provide enough water to maintain a moist soil. 




How To Prune a Clethra

Clethra do not require pruning, however respond very well to it. 


When to Prune

Since Clethra bloom on new growth, pruning can be done during winter or early spring without effecting the summer bloom. Pruning is a good way to rejuvenate the shrub following a harsh of winter. Early spring, just after new leaves have begun to emerge, is usually the best pruning time. At this time, use sharp bypass hand pruners or loppers to shape as needed.

Remove damaged or dead branches as the occur by cutting them off at the point of breakage or at their origin. 

Note:  Clethra can be pruned after flowering, however, cease pruning 2 months prior to the average first-frost date in your area. 





Plant Long & Prosper!™

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