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Carol’s Favorite Summer Blooming Shrubs

This is a recap of a free talk at Behnke’s that Carol Allen gave a couple weeks ago.  If you would like to know about upcoming talks, click here.

Carol ended the talk with a discussion of proper planting techniques which I think is important enough to discuss first thing.  She was adamant that you show your new plants tough love and really pull the roots apart before planting.  Some of us just rake out the little roots on the sides, but apparently this isn’t enough.  The majority of the roots will continue to grow in the shape of the pot and the plant will be pot-bound even though it’s in the ground.  Instead, Carol suggests that you split the roots in half and flatten them out so that you form a thick disk instead of a narrow cylinder.

The other mistake people make when planting is to only amend the soil around the root ball.  In clay soils, this creates and bathtub effect in which water can’t drain from the hole you dug because everything surrounding it is nonporous clay, so it just sits in that hole with your new plant and drowns it.  Lesson is, either amend the whole garden (preferred) or nothing.

OK, enough of that, on to the flowers!


Everyone knows hydrangeas; they’re the most ubiquitous summer-blooming shrubs for shade.  They’re deciduous and, sadly, not deer-resistant, but their huge clusters of flowers throughout the summer make them a favorite.  The classic, old-fashioned mophead varieties with their large balls of flowers are the most widely recognized and planted, but Carol also talked about lacecap, oakleaf and paniculata varieties.  The paniculatas are fine in full sun, and the least shade-tolerant of the group.

With any variety, she suggests you put the pruners away and give each enough room to grow to the size and shape they want.  She said that no matter it’s labeled (dwarf, 2’x2’, etc.), it’s going to get big.  It may stay smaller longer, but make sure you do your homework and research how big it’s really going to get.  She recommended two websites that have good info on true plant sizes: Missouri Botanical Garden (an enormous library of plants with lots of info about each) and Dave’s Garden (people write in with their own experiences with each plant).  And, while there are still varieties that only bloom once from buds that grew in the fall, many newer varieties have been bred to bloom continuously through the summer, with buds that form in the fall and spring.  Long-season bloomers are more reliable; if you prune the old-fashioned types too late in the season, or the deer eat the buds during the winter, or they are killed by freezing, you have lost your flowers for next year.  The newer varieties also form buds on the current season’s growth, so even though any of the above occurs, they will still make new buds and you’ll get blooms for part of the summer.

Hydrangeas may like shade, but not all shade is equal.  The brighter the shade, the better the blooms.  Dense shade (e.g. under southern magnolias or white pines) is too much—give up on the flowers and plant some ferns.  The best kind of shade is actually some sun in the morning, but shade in the afternoon.  All flowers need some sun.  The flipside of that (shade in the morning, hot sun in the afternoon) is going to be too much for hydrangeas (aside from the paniculatas).  If it wilts, it may need water, but there’s a good chance it’s actually getting too much sun and would prefer a different location.  The ground may be moist, but the hydrangea can’t absorb water fast enough to replace the water lost to the heat of afternoon sun.  And if it’s been in the same spot for a couple years and still doesn’t bloom well, nine times out of ten, Carol says, it’s getting too much sun.

A note on changing the color of the flowers of the mopheads and lacecaps: yes, you can go from pink to blue or vice versa depending on your preference, but make sure you do it slowly!  You don’t want to burn the roots and stress the plants by adding too much stuff to the soil at a time (iron sulfate for blue and dolomitic lime for red/pink).  Carol also warns against using aluminum or potassium compounds to change the color because Maryland already has plenty of each in its soil and too much could stress the plant.

Lacecap Hydrangeas
This is Carol’s favorite.  Like a poinsettia, the colorful petals are sterile and the true flowers that get pollinated are the little balls in the center.

Oakleaf Hydrangeas
These get huge!  Even the dwarf ones.  The leaves and flowers also get huge.  This is a great plant to use as screening or in the back of the garden.  It likes part sun/part shade and will sucker some.  The flowers are beautiful dried.  They make good plants for fall too, with dramatic red fall foliage.

Paniculata Hydrangeas
OK, so these aren’t actually for shade—they like full sun—but since we were already on hydrangeas, might as well cover these.  Historically, blooms only came in white, but some new varieties feature pink flowers.  Carol warns that the pink varieties are so new, that the color isn’t a guarantee; don’t be surprised if your pink flowers eventually revert back to white.  These are less deer attractive, but once again, Carol warns: “Don’t hold your breath.”


Summersweet (Clethra)
This is a native plant that blooms in July and August with very fragrant blooms.  It’s stoloniferous, meaning it spreads by runners.  If you want to control the size, don’t prune it, instead use a shovel to dig out the bits you don’t want.  Butterflies and hummingbirds love it.


Crape Myrtle (Lagerstroemia)
Another wildly popular plant.  Breeders have been going crazy and now you can get crapes in a vast range of sizes and colors (Carol pointed out that varieties with very dark/black foliage actually hold that color, unlike many other dark-leaved plants that fade in the heat of summer).  Once again, do your homework to find out how big they’re really going to get.  Prune in early spring before buds form, but avoid what Carol calls “crape murder,” that is, cutting it back too far/chopping the tops off.  Instead, make sure you site the plant where it will have enough room to grow so that a light pruning to improve form is all that’s needed.  Crape myrtles can get powdery mildew (Carol says: “We live in the powdery mildew capital of the world.”), but don’t waste time trying home concoctions you see while browsing the web; there’s no science behind most of them and chances are, they’ll burn the foliage.  Instead, stick with good old NEEM oil or horticultural oils and spray the tops of the leaves early in the morning before temps get above 85 degrees.

Rose of Sharon (Hibiscus syriacus)
This is a great one for heat and humidity and, once again, there are many sizes and colors to choose from.  A word of warning: some varieties should be deadheaded, unless you want inflict it on your neighbors.  Look for sterile varieties if you don’t want to deadhead.

Butterfly Bush (Buddleja)
This is a bit of a controversial choice because it is on the invasive list, but Carol doesn’t think it seeds around too much here.  If it will make you feel better, there are also sterile and low-seed varieties, though some of them don’t have nectar for butterflies, so they’ll be for looks only.  The other issue that some people have with butterfly bush is the theory that the nectar isn’t nourishing to butterflies, butterfly junk food, if you will.  Carol calls this bogus, saying there’s no science behind this theory and sugar is sugar.  Old-fashioned types might need a severe cut-back in late winter/early spring to control size, but only do this to established plants.

Chaste Tree (Vitex)
Good for pollinators.  Depending on variety, you may have to give it a lot of space.  Older varieties may grow 10ft – 20ft tall, but there are new varieties that will stay much smaller and more compact.

Cinquefoil (Potentilla)
Gets covered in little yellow, white or light pink flowers and blooms all summer long.  It stays compact, so it’s great for small spaces.  Unfortunately, it does like really, really good drainage.  So if you, like most of us in the DC area, have clay, this may not be the plant for you.

Ninebark (Physocarpus)
A native that has become a breeder favorite so there are many interesting varieties to choose from.  Features white inflorescences of flowers, sometimes with a flush of pink, and foliage beyond green, that ranges from chartreuse to black.  Exfoliating bark in winter provides additional seasonal interest.

Produces a ton of flowers mid-May through September.  It stays shorter, making it good for the front of the garden.  Be aware though, it’s on the invasive list because it seeds around.

St. John’s Wort (Hypericum)
Carol likes the shrub variety much more than the thuggish perennial variety.  It gets covered in sunshine yellow, puffball flowers all summer and stays small.  Carol warns that the variegated form may not be as hardy.

Bluebeard (Caryopteris)
Features silvery foliage and fuzzy, blue flowers that range from dusty to deep blue in August.  Stays compact.  Prune in early spring.  The four-lined plant bug may come through and in a day eat a bunch leaves, but there’s only one brood per year, so don’t worry about it.

Carol likes the variety ‘Edward Goucher’ with its link pink flowers, but ‘Kaleidoscope’ is a nice smaller variety.  Leaves are semi-evergreen; in a mild winter it retains its leaves.  Some varieties can get big, making them good screening plants.  Bloom time: summer through fall.

Smokebush (Cotinus)
Some people call it “Smoke Tree” because it gets to be the size of a small tree.  It has unusual plumes of flowers that make it look like it’s covered in a smoky haze.  Leaves are also nice, coming in chartreuse or purple.

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