Lessons From Three Adjacent Silva Cell Projects

Silva Cells were installed in 2010 and 2011 at three sites (highlighted in yellow) around the sides of a building as part of the WaterfronToronto revitalization project. The trees on each site fared very differently. Why?

Silva Cells were installed in 2010 and 2011 at three sites (highlighted in blue) around the sides of a building as part of the WaterfronToronto revitalization project. The trees on each site fared very differently. Why?

The revitalization of Toronto’s waterfront is the largest urban redevelopment project currently underway in North America, and it is also one of the largest waterfront revitalization efforts ever undertaken in the world. Waterfront Toronto is a joint venture by the Federal, Provincial, and City of Toronto governments.

The Silva Cell is being used on several areas of this project, but the three I want to talk about are of particular interest because they are all adjacent to one another, abutting three sides of the Corus Entertainment building along Queen’s Quay East. You might assume that the trees on all three sites would fare similarly, given that they are located mere feet apart and were planted round the same time. Actually, all three performed very differently at the outset. Why?

The three sites where Silva Cells were installed and trees were planted are Sugar Beach, Water’s Edge Promenade, and Sherbourne Common.

Sugar Beach

Sugar Beach trees in 2011

These 33 Maples (pictured above) were planted in a two-deep Silva Cell system in April 2010. The trees are quality nursery stock and arrived on site in good condition. They were planted in late spring, prior to bud break, and have received adequate irrigation since being in the ground. This is critical, since trees can lose up to 90% of their absorbing roots when they are transplanted.

The update on these maples is short and sweet. Take a look at this picture from 2012:

Sugar Beach trees in summer 2012.

Sugar Beach trees in summer 2012.

And this one from June of this year:

Sugar Beach trees in 2013

Sugar Beach trees in 2013

They all look great.

Designer Marc Halle, with Claude Cormier + Associes, said of them:

“I recently visited the trees at Sugar Beach – they look like they are on steroids – phenomenal growth that I have never seen before for an urban tree.”

These trees have all exhibited strong color, new growth, and overall vigor from the outset, and barring unforeseen pests or damage, their expected outlook continues to be very positive. This project won an ASLA Professional Award in 2012.

East Bayfront Water’s Edge Promenade Dockside

These 43 Red Maples were planted throughout fall of 2010. Their initial growth wasn’t great, but that is common for trees in transplant shock. Young trees and transplanted trees both have high water requirements, and a lack of adequate water from the outset will make it very difficult for them to survive and thrive.

Water’s Edge Promenade trees planted in Silva Cells in summer 2011.

This site has two irrigation systems: one within the small circular tree opening, and another one within the Silva Cell system. What we discovered after noting the poor condition of the trees is that the system under the pavement had not been activated after the trees were planted. In addition to this, there did not appear to be monitoring and hand watering of the root balls. Finally, the bubbler tree root feeder was probably specified to be a type that was overly deep, meaning that water was being added only to the lower soil levels outside the root ball rather than the upper soils, where most of the fine absorbing roots are. Not much water was migrating between the soil outside of the root ball and the soil in the rootball itself.

In addition to this, a soil test showed that the specified soil was a little low in Nitrogen, a macronutrient essential to photosynthesis and healthy tree growth. James Urban, the soils consultant on the project, requested a fertilization program to correct the deficiency. The irrigation went back on before the system was winterized that year.

Here are the trees as of this June. They have recovered well and appear full and lush.

Water's Edge Promenade trees planted in Silva Cells. Photo from June 2013 (three growing seasons). Image: Claude Cormier + Associes.

Water’s Edge Promenade trees planted in Silva Cells. Photo from June 2013 (three growing seasons). Image: Claude Cormier + Associes.

Sherbourne Common

Beech and Oak trees were planted at Sherbourne Common throughout spring, summer, and fall of 2010, and in to spring 2011. Silva Cells run under the entire width of the pavement on both sides of the park. As you can see, the trees didn’t look great at the outset. In this photo from 2011, their leaves are unevenly distributed and the branches have areas of dieback. You’ll notice that the same stock in the open planters to right are suffering from similar problems. Based on that, we determined that this is not a soil volume or soil type issue.

These trees all arrived from the same nursery in poor condition, many with co-dominant stems and other undesirable physical characteristics. Poor quality nursery trees are less tolerant of transplant shock and establishment stress, further predisposing them to other complications like pests and windthrow. Michael Ormston-Holloway, a landscape and urban ecologist with The Planning Partnership, inspected these beeches after the 2011 growing season. According to his report, most of the trees appeared to be struggling and suffered from increasingly severe dieback.

Cottony Maple Scale (Pulvinaria innumerabilis) was detected in the trees in 2011. It has since cleared up.

The first year they were planted, the Sherbourne Common trees were found to have Cottony Maple Scale (Pulvinaria innumerabilis). With attention from an arborist, it cleared.

Ormston-Holloway’s report explained that the dieback could be explained by several factors, among them: transplant shock, depth of planting, tree care prior to installation, and nursery conditions. In addition to these factors, a pest – Cottony Maple Scale (Pulvinaria innumerabilis) – is also now present. All of these factors combined caused a “serious decline in vigour.”

Based on this, Ormston-Holloway prescribed a horticultural oil to be applied to the trees prior to bud burst in the spring to control the pest, as well as a foliar spray when the pest was active in early July. The recommended treatment disrupted the lifecycle of the Cottony Maple Scale and enabled these young trees to regain their strength.

The Oak trees, which do not have a pest problem, also stabilized and recovered. Today, their leaves look healthy, indicating that we should see continue to see good growth and vigor in the future. Still, these trees have poor branch structure and other physical deficiencies which can have long term effects on tree health.

The trees at Sherbourne Common. Much improved, but not out of the woods.

The trees at Sherbourne Common in 2013. Much improved, but not out of the woods.

The variability of the tree performance on these three sites highlights some important fundamentals of planning for successful long-term growth:

Start with good nursery stock; poor quality stock can predispose trees to secondary problems such as pest infestations, slow growth, instability, and more.

Transplant at an appropriate time of year and make sure that trees are planted at the proper depth, with the root flare exposed.

Plan for adequate irrigation in the first three years; the water needs of young and transplated trees are high until they are established.

Follow up to diagnose trees that are failing to thrive, and make appropriate recommendations to correct any deficiencies or problems.

These are basic recommendations, yet they can be challenging to achieve on any project.

These three sites are interesting to compare because of all the things they share (proximity, climate, construction conditions) as well as the ones they don’t (nursery stock quality, planting timing, irrigation). Assessing their condition involves taking both factors in to account. Many different organizations and individuals are involved in the success of a single tree – the designer who puts it in the plans and designs its environment, the contractor who builds the site, the nursery that provides the plant stock, the sub who may actually be tasked with putting it in the ground, the maintenance crew or groundskeeper responsible for upkeep. Yet they are often completely isolated from one another, making it very hard to ensure that landscape plants are positioned for success from the outset.

We expect that the trees on these three sites will continue to perform well, and we’ve been thrilled to be a part of WaterfronToronto’s transformation of this part of the city. Everyone involved in construction and development has the opportunity to influence the life of the site’s trees, no matter where they are located. Perhaps that is the real and ongoing lesson.

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