Trolley Tours and Planetrees

Platanus x acerifolia is commonly called London planetree

On Saturday, September 29th, Melissa Fisher, Director of GreenBranches, and I led a tour for supporters of Friends of the High Line. In a traditional trolley provided to us by our friends at New York City's Department of Parks & Recreation, we took a great group on a tour of gardens in Red Hook, Brooklyn. We visited the GreenBranches Learning Garden at the Red Hook Public Library, the new Red Hook Senior Center Garden installed by John Cannizzo and the GreenTeam, and then finished with a stop at the Added Value farmers market for some fresh, locally grown produce. The goal of the tour was to show people how the creation of beautiful, sustainable green spaces can foster community development and improve our quality of life in the city. Much the same way the Friends of the High Line are going to improve the neighborhood and quality of life for those that live near the elevated rail over on the West Side of Manhattan.

Being that trees are one of my biggest loves in the world of horticulture, I wanted to share with you something that I learned recently about the London Planetree, one of the most widely planted street trees here in New York City. I had learned years ago that one of the reasons London planetree is such a successful street tree is because it was one of the trees that survived the coal-driven industrial revolution in Great Britain. It is true that these trees can fall victim to a number of issues such as anthracnose, canker, and powdery mildew, but their hearty appetite for Carbon Dioxide makes them quite pollution tolerant and a favorite for urban arborists everywhere. In addition, I have always admired the exfoliating bark of these trees, but never thought about how that characteristic might help with their pollution tolerance. That is, until I got chatting with a tour member named David, a New Yorker by way of the UK. We got chatting about the amount of soot and grime that was in the air during the industrial revolution and how the exfoliaiting bark helped to shed away those layers of filth that would collect on the trunk. A tree that actually cleans itself by shedding it's skin once it's too dirty and not respiring enough, brilliant! And here I was leading a garden tour showing people a whole new part of the city they had never seen before and educating them about HSNY and I end up with the one of the best horticulture lessons of the day. That is why I love horticulture. It is a science that allows you to constantly be learning about the world around you.

Best Time to Prune Smokebush

(A recent photo I took of Cotinus coggygria 'Royal Purple')

Question:
I have a few smokebush that have gotten really leggy and unattractive. What is the best way and time of year to prune so that they appear more bushy and full?

Answer:
Smokebush (Cotinus coggygria) is a favorite deciduous shrub of mine and certainly a fabulous plant for the city. With its rounded leaf, unusual bloom that resembled a cloud of smoke, and attractive fall color, this shrub is great in groupings or alone in full sun to part shade. However, I too can relate to how they can get leggy if they have been unkempt for a number of years. If this has occurred, then you can prune the shrub heavily and over a few years you can help the plant to regain a more attractive form. Before I go on, keep in mind that because Cotinus bloom on wood that is a couple years old, you may sacrifice blooms for a season or two. However, you will end up with a shrub that you are once again happy to look at in the landscape and in time the flowers will come back bigger and better than ever.

The best time to begin pruning your Cotinus is in spring before it pushes out new growth. Inspect your plant at its base and identify three to five strong stems that make up the main structure of the shrub. Prune those stems back to two or three feet above the ground. Space out your pruning cuts selectively so that you are pruning every other, or every third stem. This allows you to begin to shorten the shrub without it looking too lop-sided or beaten-up. At this time you can also cut out any weak growth from the base to promote new stems for that growing season. The branches that you have cut should push out new growth during next summer that is nice and full of foliage. The following year, revisit your Cotinus and target the next third or half of leggy stems. I would choose to cut those stems back hard as well, again, selectively, pruning them to any where from 18” to 3’ or 4’ above the ground. Depending on the size of the shrub you might be able to rejuvenate your Cotinus in two years. If it is very large it might take three or four years of selective pruning to get it back to a desired shape. Eventually you will have brought all of the major stems of your shrub back to similar heights and the new growth from all of those cuts will give you the full, foliage-rich Cotinus that you have been missing. Continue to mulch and irrigate your shrubs regularly to promote a healthy plant and you will see large plumes of that pink “smoke” flower again in two years time.

As far as Cotinus obovatus, commonly called smoketree, personal experience and some research here in the library confirms that the tree form of Cotinus does not require the same pruning regiment as smokebush.

Recent Photography: Reliable Plants

On September 20th, I gave an evening talk free to HSNY members on a number of reliable plants for city gardens. The talk was part of HSNY's educational programming in horticulture. It consisted of 20 plants that we have had good experiences with at our GreenBranches Learning Gardens, not to mention a few others that I have seen do very well in urban gardens throughout New York City. For my talk I mostly used photographs from HSNY gardens. However, I also visited my alma mater, the New York Botanical Garden, so that I could photograph a few specific cultivars to show that within a species you can find many options. Here a few of those plants:Sedum 'Autumn Joy' is a sun loving, drought tolerant 2' perennial with spectacular clusters of star shaped pink flowers late summer. It is often followed by attractive fall color.

Heuchera cultivars come in many different colors, from green to maroon to purple to silver and combinations thereof. This is Heuchera 'Pewter Veil' planted under a small tree in part shade. This mounding 12"-18" perennial I love as an edging plant or mixed with low-growing grasses to provide contrast in the garden.
Yucca filamentosa is a North American native and adds instant 2'+ structure to either a perennial or annual display. Since it is so spiky I usually mix it with hot colors like yellows, reds, and oranges. When I worked for the New England Wild Flower Society there was an all-native planting of Yucca filamentosa, prickly pear cactus (Opuntia humifusa), and black-eyed Susan (Rudbeckia sp.) that I absolutely fell in love with. Two final words on Yucca: drought tolerant.
Heuchera 'Amethyst Mist' and variegated Adam's needle, Yucca filamentosa 'Variegata'.
Hakonechloa macra 'All Gold' is a more yellow form of the classic Hakonechloa macra 'Aureola', commonly called Japanese forest grass or sometimes hack grass. A great low, spreading grass that tolerates a little shade and mixes well with many groundcovers and mounded perennials.
Nepeta sp. is called catmint, and the foliage when crushed is quite fragrant. Pinch this mounding perennial back throughout the spring and summer and you end up with a full bushy plant with excellent fine leaf texture that blooms repeatedly all summer long.
Cotinus coggygria 'Royal Purple' puts out new stems and foliage that in full sun are spectacular. An excellent upright woody shrub for foundations or alone in the landscape, smokebush puts up a light plume of flowers mid- to late-summer that resembles a cloud of smoke.
Common crapemyrtles, Lagerstroemia indica, grow their foliage and late-summer flower clusters on top of long trunks that are exceptional for their structural look and bark texture. Crapemyrtle cultivars can be classified by overall size, flower color, trunk color, and fall color. Talk about options! And as global warming makes our winters more and more mild, these attractive trees from down south are slowly showing their viability in New York City. I even know of an estate manager creating his own hybrids that are tolerant of salt spray coming off Long Island Sound. Lagerstroemia indica 'Choctaw', photographed above, is a tall tree to 33', with clear bright pink flowers, light to dark cinnamon brown bark, and bronze-maroon fall color.
Cornus sericea is commonly called redosier dogwood. Planted in full sun this woody shrub can grow to 6'-8'. The new growth comes out a bright fire engine red, an extra color element in the winter landscape especially. I also like in spring when the new leaves are more chartreuse and contrasting the bright red stems. I would take selective branches and cut them back hard every couple of years to keep that red new growth coming back.
Cercis canadensis is another North American native that is a favorite small tree of mine. The bright pink/magenta flowers that bloom right on the dark stems in spring followed by heart-shaped leaves make it a great tree option for small city gardens. I'd plant this tree in a sunny protected courtyard in a second. The culivar 'Forest Pansy' has new growth that is more red and matures green as the tree ages, granted it is in full sun.
Most people know the glossy pachysandra that is all over the city, Pachysandra terminalis, but the above plant, Pachysandra procumbens, is the North American native alternative to the glossy form. Whether it is classified as being simply "aggresive" or more seriously "invasive", I'm not a huge fan of Pachysandra terminalis because it can easily take over other plants in the garden if left unattended. Therefore, I like the larger leaf and more mat-finish of Pachysandra procumbens which still spreads well in part shade and has similar clusters of sweet white flowers in early spring.

Plant ID: Abelia x grandiflora

One morning I was heading to a meeting in Williamsburg, Brooklyn, and came across this glossy abelia (Abelia x grandiflora) planted in what looked like a tree pit. Even though the above plant might not be the prettiest specimen I have ever seen, glossy abelia is an excellent shrub and worth mentioning.

Let's look at the above conditions. Planted in a tree pit we know the plant has limited root space. Since it is out on the curb I am going to assume that it doesn't get supplemental irrigation aside from rain storms. The plant is in full sun, perhaps part shade for some of the day. Being on the edge of a large street I know it is exposed to wind and pollution. And yet, this plant is holding up very well. I think with some pruning out of deadwood and/or dieback this dense multi-stemmed shrub would be a beautiful specimen.

Glossy abelia is a wonderfully durable and reliable shrub. It grows best in zones 6-9. It is not the showiest plant in the northeast, but it can provide medium structure (in terms of size) and medium texture (in terms of the leaf size) to any green space. It mixes well with either perennials or trees and evergreens, or both. This picture is a close-up of the same plant. You can see here how attractive the foliage can be if kept pruned and neat.
I have used glossy abelia mostly as a foundation planting, both in the country and the city. It grows comfortably to 3'-6' high and wide. Once established I have seen the plant tolerate some drought. Perhaps one of my favorite characteristics about this plant is it's long bloom time. Small white tubular flowers are borne in clusters starting in June and can last for months. Even after the flowers pass the shrub holds on to it's reddish sepals which add an extra bit of color and texture. These pictures were taken mid-September and the plant was still covered in blossoms, not to mention pollinators. It is important that we pay attention to pollinator populations as they tell us a lot about the state of the environment.

Notes from Around Town: Green Walls!

I am sure that most of you have heard of green roofs, but have you heard of green walls? These pictures I took recently in Williamsburg, Brooklyn, of a new green wall on N. 4th between Bedford and Driggs. My boss lives in the neighborhood and she made the discovery one morning while walking her dog. She gave me the tip and I had to check it out.

I have been studying green roof technology here in New York City for a little while but this is the first I had heard of an exterior green wall here in the city. The purple flower in the bottom photograph looks to me like a kind of Delosperma, a genus of succulent species in the Aizoaceae family that produces a flower that novices might think looks like an Aster. Looking at the other plants in the wall I found that the majority of them were different types of Sedum. Sedums are also cold-hardy succulents, members of the Crassulaceae family. Sedums have become very popular in recent years due to their success and durability as green roof, and now green wall, plants. Because succulents use their leaves for storage of water and carbohydrates more so than their roots, they can often live and thrive growing in much shallower soil. With roots that only require a few inches of growing medium to properly anchor themselves and allow for water uptake, sedums can store enough water and carbohydrates above ground and can tolerate weeks of drought. People often think that regular rainfall is enough water for us to grow our favorite plants, but the reality is that many need supplemental water. Growers have been doing trials of Sedums for many years now and have proven that they have some of the best drought tolerance. As a bonus, many sedums have excellent fall color so I am very excited to see how this wall evolves through the seasons. One such grower who has become a real guru here in the States is a gentleman named Edmund Snodgrass who wrote a book entitled Green Roof Plants: A Resource and Planting Guide. This book and others on green roof and green wall technology are available to be viewed by the public free of charge here at our HSNY Library.

This green wall I think is beautiful and if you are ever in Williamsburg I suggest you go to N. 4th Street and check it out. There are companies nationally and internationally that sell the different modular components to construct green walls and I am constantly learning more about who they are and what they offer. As these green technologies begin to catch on more here in the United States The Horticultural Society of New York continues to be a knowledgeable and reliable resource for all. This fall there is going to be a full-day symposium on green wall construction here in the city and if you are a developer, landscape architect, landscape designer, or horticulturist, this might be a great opportunity to add this up-and-coming green technology to your repertoire. For more information, feel free to email me your questions any time.

Plant ID: Physaria didymocarpa

Question:
I found this succulent while hiking on Mt. Assiniboine this summer in British Columbia, Canada and am very query as to what it might be called. I can not find it in the few books which I have. Would you be able to assist me in identifying this succulent?

Answer:
The plant you are trying to identify is called Physaria didymocarpa, commonly called a bladderpod. Even though it does have many characteristics of a succulent, it is actually a short-lived alpine perennial belonging to the Brassicaceae family. From the rosette of silver-gray leaves this plant produces small, bright yellow flowers in summer. These flowers are then followed by the inflated seed pods that you found and photographed while on your hike. I believe that it is native to Western North America, but I am still confirming that. Physaria didymocarpa typically grows in gritty, sharply draining soil in full sun, and is considered cold-hardy in Zones 3-8.

How to Combat Whiteflies

Question:
Can you suggest the best methods to get rid of white flies, both in a New York City garden, and then as a precaution before bringing plants indoors for the winter?

Answer:
Sure enough white flies are one frustrating little pest, aren’t they? The main problem that sets white flies apart from the other insect pests is that they can produce so many generations a year. According to reference books here in our library, most species require 20-30 days for a complete life cycle, and that length of time can be even shorter during the warmer summer months. With numerous overlapping generations in a year infestations can happen quickly and can be very overwhelming, but I am more than happy to share with you my tips for helping to control them.

First, as I like to do with all insect questions, let me describe the pest and the damage so you know more clearly what you are looking at. Whiteflies are minute sucking insects that hide and feed on the undersides of leaves. If you shake a plant with whiteflies you will see them fly up and make a little fluttering cloud. Adults lay eggs on the undersides of leaves and these eggs are gray or yellow and roughly the size of a head of a pin. The larval nymphs emerge only a few days later as translucent little scales, also on the undersides of the leaves, and begin feeding off the plant. As they suck out the plant cells to feed they continue to go through different growth stages until they become adults only a couple weeks later. As you might guess, this feeding weakens and stresses out the plant. A good tell-tale sign of whitefly damage is the sticky honeydew that they secrete and leave on the leaf surface. This sugary honeydew can lead to sooty mold, a black fungus that grows on the honeydew covering leaves and fruit. Sooty mold is not likely to kill your plant, but it sure is ugly.

If you have whiteflies on plants that you still have outside, I would go ahead and treat the plants with an insecticidal soap right away. Safer brand insecticidal soaps and other such soaps you should be able to find at your local garden center or any larger retail store (Lowe’s, Home Depot, etc.). Read the instructions and spray and coat the infested plants as best you can. Remember that spraying the undersides of the leaves is crucial since that is where whiteflies spend most of their time. I believe the bottle will recommend spraying the plant to the point of runoff. As long as the leaves of the plants are not too fragile, like certain ferns might be, you can repeat application of the insecticidal soap weekly until you see the situation getting better. In addition to spraying, any physical removal of eggs or nymphs that you can do will be a big help as well. A soapy paper towel and/or swab with rubbing alcohol will help as you attempt physical removal.

Finally, when you move the plants back inside before frost, put out whitefly traps. You can buy them prepackaged or you can make your own. Get a piece of florescent yellow paper and cut it down to 2” x 2” squares. Coat the paper with petroleum jelly. Then use a cut-down plant stake, or some such support, to place the traps in your containers. Since they are not the most aesthetically pleasing, place them in the back of the pots so they are not such an eyesore. The bright color attracts the whiteflies and they get stuck to the petroleum jelly. Throw out and replace as necessary. This way you can keep track of the pests through the winter and you can tell if there are enough of them that another application of insecticidal soap is necessary.

Containers for a Brooklyn Brownstone

Question:
Would you know a store/nursery in New York City for stoneware or cement rectangular planters? Since these planters are for the front exterior large window ledges of a brownstone in Brooklyn, I am looking for planters in a classic design ( e.g., fluted, bas relief, etc). I would also consider planters in faux terracotta/cement, as long as it is not noticeable that they are fakes.

Answer:
I would first suggest the new Chelsea Garden Center location out in Red Hook, Brooklyn. The address is 444 Van Brunt Street, (adjacent to Fairway), Red Hook, Brooklyn, 11231. A coworker of mine who lives in Brooklyn said that she found a very large selection of planters there last time she went by. In addition, if you are a member of The Horticultural Society of New York you can present your membership card for a 10% discount on your purchase(s). The other Chelsea Garden Center location is at 580 11th Avenue at 44th Street here in Manhattan, but again, Red Hook should have an equal or better selection.

Otherwise here in Manhattan I might send you to 28th Street to the flower district. Jamali Gardens (149 W. 28th) between 6th and 7th Avenues has a number of large planters that are reasonably priced, but they might not be as decorative as you’d like. There are other shops on 28th that sell large planters, but I fear most of them now only sell wholesale.

Street Trees in New York City

(The larger tree with the fluted growth habit is a Japanese zelkova, or Zelkova serrata. I love these street trees for their fall color that can range from bronze to russet to rich maroon.)

Question:
Would you happen to know what most of the trees are on the streets of New York? Someone told me that they were mostly Gingkos but I don't see a lot of them around.

Answer:
That is a pretty involved question but I will do my best to give you as much information as I can. Luckily for both of us I know a number of individuals who work for the Department of Parks & Recreation here in the city and I have been involved in many discussions and workshops about the present-state and future of street trees here in New York City. Most of the information I am about to share was provided to me by Parks & Recreation and if you would like to visit their website, I have created a link here.

The City of New York Department of Parks & Recreation performed a city-wide street tree census from 2005-2006. For the census they defined a street tree as any tree growing within the public-right-of-way, or within 15 feet of the curb. With the help of over a thousand volunteers, Parks & Recreation determined that there were a grand total of more than 592,000 street trees growing in New York City. This census showed a 19% increase in street trees since the previous census performed from 1995-1996.

According to a summary put out by Parks & Recreation, census-takers identified a remarkable 168 different species of street trees. Furthermore, it was found that 74% of the overall population of street trees was comprised of only ten specific species. Here is a list of the top ten street trees in New York City for 2005-2006:

London planetree (Platanus x acerifolia) 15.3%
Norway maple (Acer platanoides) 14.1%
callery pear (Pyrus calleryana) 10.9%
honeylocust (Gleditsia triacanthos) 8.9%
pin oak (Quercus palustris) 7.5%
littleleaf linden (Tilia cordata) 4.7%
green ash (Fraxinus pennsylvanica) 3.5%
red maple (Acer rubrum) 3.5%
silver maple (Acer saccharinum) 3.2%
ginkgo (Ginkgo biloba) 2.8%

As you can see, ginkgos are low on the list but still one of the most common street trees in New York City. It is also important to keep in mind that this information includes all five boroughs and may not be an exact representation of your neighborhood. In some neighborhoods we inevitably find higher populations of certain trees compared to others. For example, in my neighborhood in Queens I know we have mostly lindens, yet when I visit my brother down in Brooklyn I typically see only honeylocusts.