A Rare Azalea

Have you ever seen an azalea blooming in August? It is a rare sight, indeed!

Plumleaf Azalea (Rhododendron prunifolium) is the latest-blooming of all North American azaleas. It starts blooming in late July or August, months after other azaleas have finished, and it has been reported still blooming in October!

Plumleaf Azalea on August 14

It is also the rarest of North American azaleas, native to only two states in the US. In the wild, a few small populations grow in forested sandy ravines along streams that drain into the Chattahoochee River, on the border between Georgia and Alabama. Sadly, even those few existing wild populations are threatened by development and careless logging practices, so the only native habitat of this amazing shrub is rapidly disappearing. Rhododendron prunifolium is under consideration for Endangered Species classification, but is not yet on the official list.

Plumleaf Azalea is strikingly beautiful. Its flowers are a deep red-orange with extra-long stamens that curl outward like exaggerated false eyelashes. The flowers are not particularly fragrant, but bees do visit them.

Long red pistil and stamens protrude from the blossom
Shiny, dark green leaves show off each cluster of flowers

Despite its very limited native range, the shrub does very well in gardens throughout the Eastern US in Zones 5 to 7. It prefers rich, acidic soil, even moisture, and dappled sun. It has a rounded, fairly compact shape, and will reach 5 to 8 feet tall and 3 to 4 feet wide. It is an absolute show-stopper in a shady garden when nothing else is in flower.

Plumleaf Azalea in a woodland garden

A plant as rare as Plumleaf Azalea poses a challenging question for ecology-minded gardeners. If it is native only to a limited area in Alabama and Georgia, should we be planting it in Northeastern gardens? There are several considerations:

We know that plants introduced into areas where they are not native can become invasive and cause ecological harm. (See our blog posts on very aggressive invasive plants introduced from other continents: Nip ‘Em in the Bud!, Evil Ivy Over Everything, and Boo!) While there are some native US plants that have become a nuisance when introduced outside their native range to other parts of the US, the danger is far less than with plants introduced from other continents. Plumleaf Azalea does not spread itself by the roots, and is not easily spread by seed, so it is not likely to move into areas where it is not intentionally planted.

But we also know that the value of any given plant to an ecosystem is greatest within its native range because it will have co-evolved with the other plants and animals in that ecosystem. A Colorado Blue Spruce, for example, supports insects native to the Rocky Mountains, but is of little use to most insects in the Mid-Atlantic region. Similarly, we can imagine that insects native to the Northeast may have no use for a plant from Alabama, though insects that evolved along the Chattahoochee River may depend upon that plant for survival. It is not known whether Plumleaf Azalea is a host plant for insects outside its native range, or whether it is useful to pollinators in the Northeast, but we do know that plants native to the Northeast are more valuable to local insects.

For these reasons, the best ecological choice is to plant species native to your specific region. But what if a plant is endangered in its own native region?

Michael S. Dosmann, writing for the Arnold Arboretum at Harvard University in 2022, noted that shortly after the founding of the Center for Plant Conservation in 1984, the Arnold Arboretum began collecting seed from disappearing populations of Plumleaf Azalea in three counties in Georgia. The goal was to prevent the extinction of the species. As of last year, the Arboretum was growing 34 Plumleaf Azalea shrubs – in Massachusetts. He said “preserving wild populations remains the highest priority, but it is important to have a back-up…”.  He also observed that Plumleaf Azalea looks great in the garden, and for endangered species, “being charismatic and attracting attention is a gateway to its security (just look at the giant panda)!” Does planting Plumleaf Azalea in our gardens provide additional “back-up” for conservation of an endangered species? Maybe…

A rare flower

With that justification, anyway, growing a beautiful rare azalea that flowers in August (!) is practically a public service!!

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The Sweetest Scent of Summer

The intoxicating perfume of Summersweet (Clethra alnifolia) is made for the sultry days and balmy evenings of late summer. 

A native shrub that blooms from mid-July through August, Summersweet has a scent like honey and cloves — not as heavy as gardenia, but spicier than lilac. Another common name for the shrub is Sweet Pepperbush, which hints at the spicy notes in its heady fragrance.

 Summersweet (Clethra alnifolia) in bloom

Summersweet blooms in large fluffy panicles of white flowers that open gradually from bottom to top. The blossoms are carried upright over shiny dark green leaves. Each stalk has dozens of individual flowers with protruding stamens offering pollen, and tubular petals surrounding a supply of nectar. Summersweet is a buffet for butterflies and bumblebees.

Flowers open from the bottom to top
A red-banded hairstreak butterfly sips nectar

Summersweet is native to the East Coast of the US from Maine to Florida. Although its native habitat is swamps and moist forests, it is adaptable, and does well in average garden conditions in Zones 4 to 9. It will grow in sun or part shade and average soil, though it prefers consistent moisture. It requires no special care, and is deer and rabbit resistant. It’s an excellent choice for low wet areas and rain gardens.

A bumblebee gathering pollen on Clethra
Two butterflies share the bounty with a bumblebee

Clethra is a suckering shrub, which means that it grows outward from its base, sending new branches along the ground where they will root and begin producing flowers within a season or two. The shrub can easily be pruned to confine it to its original spot by trimming off the suckers as they appear, or it can be allowed to gradually form a colony or thicket. In most conditions, the shrub will grow to 5 or 6 feet in height and about as wide, though it can reach 8 or 9 feet. Tall stems can be cut back in the fall without risk of losing spring flowers.

At the Greenburgh Nature Center, a 15-year old Clethra has reached the eaves of the Manor House at about 9 feet, while 4-year old shrubs are still only 3 or 4 feet tall.

The nursery trade has been busy developing cultivars of Clethra alnifolia over the past few decades, so there are good choices for every home garden, and all of them are fragrant. The dwarf cultivar ‘Hummingbird’ only grows to about 3 feet tall and wide, while ‘16 Candles’ tops out between 3 and 5 feet and produces abundant flowers. Another cultivar called ‘Ruby Spice’ produces lovely pink flowers, though it is unknown whether the change in flower color adversely affects its value to pollinators.

The leaves of Summersweet are hosts for at least 11 species of moths and butterflies, which is just one reason this plant is a much better choice than so-called “butterfly bush” (Buddleia sp.). Butterfly bush is not native to North America, and has become invasive in many areas because it produces vast amounts of seed while providing very little other value to native insects and birds. Butterflies will drink its nectar, but their larvae cannot survive on butterfly bush. Summersweet is the perfect native alternative, and you will find it widely available at nurseries.

A silver-spotted skipper nectars on Summersweet

Summersweet leafs out very late in spring, so don’t worry if you still see bare branches well into April. On the other hand, its fall color is an attractive bright yellow, and when the leaves finally drop, interesting seed clusters remaining from spent flowers provide winter interest.

It’s not too late to add Summersweet to your fall planting list! It would be a perfect choice near a deck or patio where you can enjoy its heady aroma while watching butterflies sip its nectar. You can be sure you won’t be the only one in the neighborhood to appreciate summer’s sweetest scent!

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Nip ‘Em in the Bud!

It’s weed season. Mid-summer heat and rain cause weeds to come up everywhere. Uggh!

But weeds are not all equal. Some of them can be ignored for a bit while you enjoy a little summer relaxation. But others are so aggressive and so destructive they should be eradicated on sight. Can you recognize the worst culprits as soon as they emerge?

There are over 1000 invasive plant species in the US. These are non-native plants that have spread themselves widely because they have no natural controls on this continent. They cause enormous harm when they take over natural areas, displacing native plants, and often ruining ecosystems. 

Each region in North America has its own “worst enemies” among invasive plants, so any short list will be too limited. But learning to identify the worst plant villains in your local area is worthwhile, especially so you can take action when they first appear. 

So, here are “baby pictures” of a few of the worst culprits to watch out for in our area (Southern Westchester County, New York) and in much of the Northeast and Mid-Atlantic regions:

Mugwort (Artemisia vulgaris)

Mugwort is a perennial herb introduced to North America long ago, probably by settlers who valued its medicinal properties. It spreads very aggressively, mostly by underground runners. Bits of Mugwort root can be transported in soil in nursery pots, transplanted trees and shrubs, sod, purchased topsoil or fill, on lawnmowers, and even in compost. Any root fragment in the ground will sprout new plants, so when you are weeding, roots must be dug out completely and repeatedly. Left alone for even a season, Mugwort will invade large areas. If (when!) you see little sprouts that look like chrysanthemum leaves, dig them out as soon as possible. They can pop up anywhere, so monitor your gardens, lawn, and even tiny cracks in pavement. The goal is to prevent them from forming new communities like this:

A Mugwort patch will become very difficult to control

Porcelain Berry (Ampelopsis glandulosa var. brevipedunculata) is a terribly destructive vine that overgrows trees, shrubs, and groundcovers, eventually killing them. It was introduced intentionally by the nursery trade as an ornamental garden plant and, shockingly, is still offered for sale. Millions of dollars have been spent trying to control this invasive weed since it has escaped gardens and now covers trees, roadsides, and open areas throughout the Eastern US. 

 Porcelain Berry looks innocent emerging through hosta

Porcelain Berry seedlings can be tricky to identify since the leaves actually change shape as they mature. Young leaves look like grape leaves, but as they mature, they become more deeply divided, with 3 long-pointed fingers and 2 shorter ones. The leaves branch from a thin stem that quickly threads itself through taller plants. 

Changing leaf forms on Porcelain Berry

Porcelain Berry seeds are widely distributed by birds who eat the berries, so you will see seedlings sprout under trees, shrubs, fences, and power lines wherever birds perch. It is important to catch the seedlings early before the vines cover everything within reach, stealing sunlight and choking trees and shrubs. Learn to identify Porcelain Berry’s young leaves, and pull seedlings out by the roots as soon as you see them.

Porcelain Berry destroying trees and shrubs

Oriental Bittersweet (Celastrus orbiculatus) is another destructive vine that also was introduced as an ornamental garden plant. Although it can look innocent as a seedling, mature vines can grow 80 to 100 feet tall, becoming incredibly thick and heavy, and often pulling down even large trees. 

Oriental Bittersweet seedling

Like Porcelain Berry, Bittersweet spreads primarily by seeds carried by birds. When the seeds germinate, the young leaves have pointed tips that become more rounded as they age. Young stems immediately begin reaching upward, making their identity as vines clear. 

Pull young vines out by the roots as soon as they appear, and cut down any vines that have reached your trees and shrubs before berries can develop. Preventing seed distribution is critical to controlling this invasive plant.

Mature Bittersweet vines loaded with berries are very heavy and threaten trees
Remove vines before berries can develop

Japanese Knotweed (Polygonum cuspidatum or Fallopia japonica) has invaded 42 states in the US, including large areas of open meadows and wetlands, overwhelming native plants and destroying essential habitat for birds and other animals. It, too, was introduced intentionally as an ornamental plant and for erosion control on highways. It spreads both by seed and by underground roots and should be destroyed the minute it appears in your garden.

Japanese Knotweed must be dug out by the roots or cut constantly until it dies from lack of photosynthesis

Its large, heart-shaped leaves are easy to spot, and its stems form a characteristic zig-zag shape. In late summer, large fluffy white flowers appear on top of stems that can reach 5 to 7 feet tall. But you really don’t want to let it get anywhere near that size. If you are unlucky enough to have a large stand of Japanese Knotweed on your property, you may have to call in professional help to eradicate it.

Japanese Knotweed grows very tall and very fast, overcoming any other plants

If you find any of these plants on your property, zero tolerance is the best policy. Pull them out with the roots while they are small and monitor for re-growth. Be sure to dispose of the weeded plants in the trash, not in compost or weed piles where they may survive to germinate again. Avoid sharing garden plants from areas where you know these weeds have been present.

Though use of chemical herbicides is usually a bad idea for human and environmental health, for big infestations of these particular weeds, you may have to resort to targeted professional applications of specific herbicides. Consult experienced tree care and landscape maintenance professionals and your local agricultural extension service for recommendations. Routine seasonal applications of herbicides on lawns and ornamental shrubs cause far more harm than good, and are not particularly effective against these most difficult invasive plants.

Weeds are a fact of life in even the best-kept landscapes. Knowing which ones can be tolerated and which cannot is important. Be vigilant, and nip ‘em in the bud!

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Monarda: A Balm for the Spirit

In early summer as bees, butterflies, and hummingbirds search for nectar and pollen, they find incredible bounty in Monarda – a group that includes some of our most beautiful and valuable native perennials. Watching wildlife enjoy Monarda on a summer day is only part of the pleasure – Monarda has other gifts for humans as well.

The Monarda genus is “endemic” to North America, meaning it is native only to this continent. There are over 20 species of Monarda, and 3 of them, all known variously as “Bee Balm,” “Wild Bergamot,” and “Oswego Tea,” make great garden plants in the Eastern US. All three are colorful, sun-loving, easy to grow, and deer resistant. The pom-pom flower heads are actually clusters of individual tubular flowers loaded with sweet nectar that attracts hummingbirds and a huge variety of pollinators. Flower colors range from pale lavender to pink to flame red.

Monarda fistulosa and a happy bumblebee

Monarda fistulosa (most frequently called “Wild Bergamot”) stands 4 to 5 feet tall and blooms with showy lavender flowers from July through September. It is tolerant of poor soil and can handle periods of drought. A true meadow plant, this Monarda is native to most of Southern Canada and almost all of the US east of the Rocky Mountains.

Monarda fistulosa in a Midwestern prairie
Equally happy in a suburban garden, Monarda mixes well with other full-sun perennials

Monarda didyma (most often called “Bee Balm”) is typically bright red, though there are natural variations and cultivars in different shades of rose and pink. It is shorter than Monarda fistulosa, usually topping out at 3 to 4 feet. It prefers more moisture and is a more aggressive spreader. Monardas are in the mint family and spread by rhizomes as well as seed. While M. fistulosa is less aggressive, Monarda didyma can take over a small flower bed sending runners in every direction, so plant it where it has room to roam. It will form a stunning colony in the right place, with full sun and fairly rich soil. Hummingbirds find it irresistible.

Monarda didyma is typically bright red
A pink variety of M. didyma
A colony of Monarda didyma at the Greenburgh Nature Center

Monarda bradburiana is native to the warmer climates of Southeastern and Southcentral US, but is hardy in Zones 5 to 8, so it does very well as far north as Connecticut, New Jersey, and parts of New York. It is a great garden plant because it is a clump-former, not spreading aggressively by rhizomes, and grows only 1 to 2 feet tall. It blooms in June, earlier than other Monardas, and has interesting shaggy flowers in pale pink.

Monarda bradburiana
M. bradburiana with Prairie Dropseed at the Nature Center

The common names for these Monarda species seem to overlap. All of them are called “Bee Balm” by many gardeners. And while the name “Wild Bergamot” most often refers to M. fistulosa, it is frequently applied to M. didyma as well. And both M. fistulosa and M. didyma are commonly called “Oswego Tea.” Though botanists would wag a finger and say this is exactly why common names are confusing, all three of these names actually are appropriate for all three plants. 

“Bee Balm” refers to an ancient medicinal use of Monarda. Indigenous Americans taught European settlers that applying shredded (or chewed!) Monarda leaves to insect bites or stings neutralizes the pain and speeds healing.

“Wild Bergamot” refers to the flavor of “Oswego Tea.” Indigenous people of the Oswego River area in upstate New York introduced settlers to a delicious tea made from Monarda leaves and flowers. The flavor reminded Europeans of the taste of Bergamot orange, the flavoring in Earl Grey tea. It is often said that after the Boston Tea Party, colonists turned to Oswego Tea and boycotted the English product. That isn’t really a sacrifice: try adding steaming water to a handful of torn Monarda leaves and let it steep for 10 minutes. It’s delicious! Add a little honey and it’s even better.

As an herbal seasoning, both the leaves and flowers of Monarda are very good. Dried leaves of M. fistulosa, which are spicy as well as minty, can be used to season roasted meats. The fresh flowers of M. didyma (pulled individually out of the cluster), have a sweet and citrusy flavor that goes well with fruit, and in salads. Though Monarda plants are vulnerable to powdery mildew if conditions are too damp, it is easy to avoid the affected leaves and select newer growth along the stem and near the flowers.

Monarda is known to have anti-microbial and antiseptic qualities. The tea helps with sore throats, mouth sores, and gastric symptoms. Poultices and salves made with Monarda are said to prevent infection and aid in healing skin wounds. Early American colonists also learned to plant Monarda around their beehives. Not only does the honey pick up flavor from the flowers, the anti-microbial effect of Monarda helps prevent diseases in the bee colony.

A balm for the spirit on a summer day…

So, try a cup of Oswego Tea, hot or iced, and watch the hummingbirds, butterflies, and bumblebees share the gifts of Monarda. It is a balm for all of us.

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Call the Marketing Department!

One of our most beautiful native perennials is commonly called “Hairy Beardtongue.” Yes, that’s right: Hairy. Beard. Tongue. 

A lovely plant with the worst common name ever!
Photo: Travis Brady

While non-native flowers, even the invasive ones, often have charming names like “lily-of-the-valley,” “forget-me-not,” “star-of-Bethlehem,” “Queen Anne’s lace,” and “periwinkle,” many of our most beautiful native perennials are tagged with ugly names like “sneezeweed,” “snakeroot,” “milkweed,” “tickseed,” “spiderwort,” and “bloodroot.” Even in that list, the awful name “Hairy Beardtongue” is the loser! We need a rebranding campaign! 

But until then, we can stick with scientific names — especially for the beautiful Penstemon genus, a large and varied family of native perennials.

A garden full of lovely Penstemons in bloom

The shape of the flowers explains why most Penstemon species are stuck with “Beardtongue” as part of their common names: Rock Beardtongue, Firecracker Beardtongue, Large-flowered Beardtongue, Rocky Mountain Beardtongue, and many others. 

Penstemon flowers typically have 5 petals fused together at the top to form a nectar-rich tube. At the open end, two upper petals curve back and three lower petals reach out to provide a landing platform for pollinators. Each flower has 4 fertile stamens bearing pollen, and one infertile stamen, covered with hairy fibers, that rests on the lower lip of the flower. Presumably, that structure is what caused somebody to think of a “bearded tongue.” Why they didn’t think of a “lady’s comb” or “baby bottle brush” instead is a mystery. The scientific name Penstemon comes from the Greek words penta meaning five and stemon meaning thread or stamen. At least that name makes sense.

Penstemon flower with so-called “beard tongue”
Side view showing fertile and sterile stamens

Penstemon is the largest genus of plants native only to North America. There are over 250 different species of Penstemon found from the northern reaches of Canada through the southern tip of Mexico. The largest number and variety of these species are native to the mountains and deserts of Central and Western North America. They all have major value to insects and birds, and most of them are very pretty.

In the Northeastern US, there are two species of Penstemon that make especially fine garden plants: the aforementioned “Hairy Beardtongue” (Penstemon hirsutus), and the somewhat less disturbingly named “Foxglove Beardtongue” (Penstemon digitalis).

Penstemon hirsutus (the word hirsutus means hairy) is identified by the many very fine hairs lining its stems. It is a wonderful addition to flower gardens: carefree, drought tolerant, deer resistant, and easy to mix with other perennials. The flowers are 12 to 18 inches tall and a soft lavender or pale pink fading to white. Lance-shaped leaves are widely spaced along the stem in opposite pairs so that the stem almost appears to be piercing through them. At ground level, there is a basal rosette, or tight cluster of leaves that remain semi-evergreen in the winter. Penstemon hirsutus is a clump-forming plant that will gradually expand at the base, occasionally spreading by seed, but it is well-behaved in the garden and suitable for smaller spaces.

Downy hairs on the stems of Penstemon hirsutus

Foxglove Beardtongue (Penstemon digitalis) is named for its vague resemblance to the popular English garden plant, foxglove, and the scientific name digitalis means finger, referring to the fingers of a glove. It grows taller than Penstamon hirsutus, reaching 3 to 4 feet, and has larger mostly-white flowers, with tiny purple stripes on its lower petals as nectar guides for bees. Its native habitat is moist meadows, but it can tolerate stretches of dry weather once established. It survives without any irrigation in the Meadow at the Greenburgh Nature Center and delights visitors in early summer. It can spread itself around more aggressively than P. hirsutus, but is not at all difficult to control. And while its namesake, foxglove, is famously toxic, Penstemon is not. Indigenous people all over North America found medicinal uses for the many species of Penstemon.

Foxglove Penstemon (Penstemon digitalis) flowers
Penstemon digitalis lights up the Meadow at the Nature Center

Both of these Penstemons bloom for weeks in early summer. They prefer full sun to part shade and are hardy in Zones 3-9. They are not at all fussy about soil conditions and are ideal candidates for low-maintenance pollinator gardens. They are both generally avoided by deer.

Penstemons form interesting seed capsules after pollination and have good fall color. They look great left standing through the winter, providing seeds and shelter for birds, and interesting form and color for gardeners.

Fall and winter interest: Penstemon in November

A popular cultivar of Penstemon digitalis called ‘Husker Red’ has purple leaves and pinkish, rather than white, flowers. Although the natural species is of more value to insects, ‘Husker Red’ mixes well with both Penstemon species described here, and adds foliage interest to a mixed flower border. 

Penstemons are of special value to native bees and are also visited by hummingbirds. Short-tongued bees will sometimes “cheat,” biting through the tubular base of the flower to reach nectar without trading pollination services. Butterflies also are attracted by Penstemon nectar, and the leaves host a number of butterfly and moth caterpillars. Bumble bees love Penstemon! As they burrow in for nectar, the “beard tongue” brushes pollen off of their bodies, aiding pollination of the flowers.

Bumblebees love Penstemon

Come to think of it, why don’t we call that fuzzy 5th stamen the “bee-tickler” instead of “beard tongue”? Call the Marketing Department! Let’s re-brand the whole genus and declare that the new common name for Penstemon is “Bee Tickle”!

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