Research Trial (Pilot Project) on the Survival and Growth of Different Sizes of Planting Stock for Street Trees in Halifax
Note #03 by Peter Duinker, August 2022
The Importance of Watering New Trees
“Both container- and field-grown plants face the same challenge initially after planting: marginal or insufficient water absorption by the root system. Until they can grow a normal spreading root system, newly planted trees are dependent on frequent irrigation. Water stress is a common cause of planting failure” (page 150 in: Watson, G.W. and E.B. Himelick. 2013. The Practical Science of Planting Trees. International Society of Arboriculture, Champaign, IL).
So, newly planted trees in the streetscape require lots of water through their first seasons to get their roots off to a vigorous start. A new tree is busy getting its root-to-shoot ratio in balance, particularly the balled and burlapped trees (see photo) which each have lost up to 80-90% of their roots when excavated from the nursery field. (The root-to-shoot ratio is the abundance of roots compared to the abundance of twigs with leaves). The roots of the potted trees have been bound up in the organic growing medium at the nursery and must venture out into the potentially hostile soils of the streetscape. Water is the key to success.
Since planting the 16 new red oaks and red maples on Lawrence St. on 25 June, I have been tuned up to their water needs. When we planted the trees, the soils were very dry. Actually, they were still dry when the six balled-and-burlapped (B&B) trees were planted on 18 July. The following table shows the total sum of water the potted trees have received from rainfall and from my watering regimen. The B&B trees experienced that same rainfall after 18 July.
The B&B trees were watered by the planting contractor on 20 and 26 July (and perhaps after that, but I have no information as yet). There is no record of how much water each B&B tree gets at each watering, but I know it is substantial (perhaps 20-40 L per visit). I use 20-L plastic jugs to get water to the potted stock, and I assume that a few litres run off the site at each watering, so that explains my 18-L estimate per watering.
Some notes of explanation. The rainfall numbers above are from the Shearwater station, except for 23 August when I installed a rain gauge at my home. I have not included rainfall events below 5 mm (or 0.5 cm), assuming they are too small for water to penetrate the ground to the tree roots. I believe this is a fair assumption given how hot it has been this summer – with evaporation from warm ground and the appetite of the grass for water, small rainfalls really do not benefit the trees except perhaps for their cooling effect which would potentially reduce a tree’s need to transpire water to cool the leaves. To calculate volume of water available to each tree from rainfall, I had to make an assumption about the appropriate area of ground to use. For lack of a better approach, I figured that half a square metre would be more that enough. Some of the holes we dug for the potted stock were much smaller than this, and the hole dug for the B&B stock would be about this size, i.e., a circle of diameter of 80 cm. If I were to assume a small relevant ground area, the relative contribution of my watering regimen would be even larger than I calculated (see next paragraph).
I examined the rainfall normals for July and August for the Shearwater, Halifax airport, and Westphal stations. The “normal” precipitation (i.e., the average over 30 years) for those two months combined is about 19 cm (I’m using cm instead of the customary mm for an easy calculation of volume). If we don’t get significant rainfall for the rest of August, that means we are in a drought situation this summer. A lower-than-normal rainfall plus the abundance of hot days in July and August means that new trees could easily become water-stressed if they do not receive supplemental watering. As evident from the table, my watering regime nearly quadruples the water available to the new trees from rainfall. A key question I’m now asking is this: have I overwatered the trees? Given the rate at which the water I give each tree disappears downward during the watering process, I don’t think so. The organic medium in which the potted trees were grown has a huge water-holding capacity but also can dry out quickly.
Until the leaves drop in autumn, I will continue to monitor the rainfall and water the trees as I deem appropriate. A rainfall event of some 30-40 mm is roughly equivalent to each of my waterings. The trees’ need for water goes down through the autumn and the rainfall amounts go up. I will do the same next summer, and by the third growing season in the streetscape, the trees should be able to fend successfully on their own. If in 2024 we get a really hot and dry summer, I will probably water them now and then. One thing is for sure – if any of the potted-stock trees dies in the next couple of years, it will definitely not be for a lack of water!
Research Trial (Pilot Project) on the Survival and Growth of Different Sizes of Planting Stock for Street Trees in Halifax
Note #02 by Peter Duinker, June/July 2022
Implementation has proceeded! Here is an update, with details on methods.
Sites
From an initial first-approximation of the number of sites potentially available, numbering about 35, the utility locates brought that down to 27. On 20 June, I put an announcement in each home’s mailbox announcing that we would put a tree in the tree lawn in front of the home on 25 June. By 24 June, five people registered an objection to having a new tree in front of their home. That brought the final number of sites to 22. Trees from the planting inventory were allocated to sites at random (using two dice – one for species and one for size!).
Stock
The order to Baldwin Nurseries was for four replicates of each of three sizes (measured by pot size – 2-gal, 5-gal, and 10-gal) of each of the two species (red maple and red oak), for a total of 24 trees. They arrived in my driveway on the morning of 23 June. All were grown from NS seed and arrived in good condition. It was clear given the variation in actual pot size and the size of the trees in them that pot size is not a clear indicator of tree size. Thus, based on measurements of diameter at root collar (about 15 cm above ground or the root flare), we will reclassify the trees into sizes based on these measurements (see below).
The balled-and-burlapped (B&B) stock came from Beech Nursery West in Schomberg, Ontario, and is of unknown provenance. All trees appeared in good condition off the truck on 18 July, except that the leaves of the red maples appeared slightly dehydrated.
Planting
The Potted Trees: The 16 potted trees were planted on the morning of 25 June. Each root ball was scored vertically four times to a depth of about 3 cm with a knife, then planted with the root flare at ground level. Each tree was mulched to about 60-80 cm diameter about 8-10 cm thick. Each tree was given 20 L water that evening. Trees were staked and tethered on the morning of 27 June. One T-bar of 180 cm length was installed about 25 cm from the stem for tethering; two wooden stakes of 1.1 m length, 4 cm x 4 cm, were also installed some 25 cm from the stem as protective guards (against urinating dogs, lawn-tending equipment, and any other potentially intrusive agents; see the accompanying photo set).
The B&B Trees: the six trees – three red oaks (Quercus rubra) and three red maples (Acer rubrum Franksred) – were installed by Elmsdale Landscaping on 18 July. The standard procedures established by HRM for installation of street trees were used for planting and tethering. Specifically, these trees were also mulched and tethered with two T-bars placed parallel to the street. A protective sleeve made from 13-cm diameter perforated drainage pipe,
about 20 cm long, was placed on each stem, resting on the ground, as protection from lawn- tending equipment. The planting contract calls for a watering regime, as needed, over the two years of tree warranty.
Watering
At the time of writing this note (25 July), I had watered the 16 trees of potted stock on 25 June, 29 June, 03 July, 10 July, 14 July, 18 July, and 22 July. Each watering involves 18-20 L of tap water per tree, gently poured within 15 cm of the base of the tree. Halifax got about 30+ mm rain on 06 July but nothing else of note (i.e., sufficient to avoid having to water the trees) since the potted trees were planted. T-max each day since planting has been near or well over 20_C. I decided not to water the B&B stock because the contractor (Elmsdale Landscaping) is obliged to water the trees as needed until the end of the 2-yr warranty period.
Measurements
As of today, I have made a suite of measurements on each tree. A table is included below. In the table you will find:
_ civic addresses on Lawrence St. in front of which the trees are planted
_ the size of pot and species of tree (for potted stock)
_ the tree’s dimensions – diameter at root collar (DRC), diameter at breast height (DBH), height (H)
_ what is above the tree in terms of other trees and power lines
_ when the tree was planted and by whom (the Dal team includes me and my summer research assistants)
Logistics and Costs
Since the experiment is looking at various aspects of planting trees of different sizes in the streetscape, I’m interested in how the costs might differ. Forthcoming will be a thorough reckoning of the costs to put in the potted trees (including tree and material costs). I will not include the labour of HRM staff in working with me to plan and execute the project because the staff also must deliver these services for contract-planted trees. And I will not monetize the value of our volunteer labour. What I can say about that is that it took six of us three hours to plant the 16 potted-stock trees, and I put in probably eight hours preparing and installing the stakes. HRM staff delivered mulch and removed excess soil, so that needs to be accounted for. Then there were materials costs including stakes and tethering wire. Finally, I put in about 1.5 hr each time I water the trees. The cost of the water (about 300 L each watering) is miniscule, and the water- delivery equipment was already in my possession. Apart from delivery of the trees and moving mulch and excess soil, no machines were run to plant and maintain the potted stock.
For the B&B trees, the planting occurred over about two hours total. The equipment list included two full-size dump trucks (one for excess soil and one for high-quality filling soil), a small digger, a small front-end loader, their respective floats, and service and traffic-control vehicles. Tethering and mulching involved another truck plus staff the next day, and the first watering also
involved a truck and one staffer. All costs associated with planting the B&B stock is bundled into a contract price of about $750/tree. Thus, the six trees cost $4,500 to install (and maintain for two years under warranty).
A Word about Tree Sizes
I had originally planned to group the potted stock according to pot size but the approach now will be to group the trees, species by species, according to diameter of the stem. Since some of the trees are too short to have a meaningful DBH, I used DRC for the size groups. Below is a histogram of all 22 trees ordered by DRC (cm on the Y-axis):
This is not the distribution I had hoped for, but it will suffice for this little experiment. I see three groupings in this dataset:
_ DRC > 4.0 cm – six trees, all the B&B stock
_ DRC range 2.1-4.0 cm – three trees (all red oaks, as it turns out)
_ DRC range 1.0-2.0 cm – the remaining 13 trees
Whether tree numbered 14 is more like its smaller or bigger neighbours is debatable – perhaps it should be 14 small and two medium. That will be sorted later.
Learning about the New Trees
My intention is to continue to post short articles now and then in this series of notes. As well, some time soon, I will advertise an opportunity for neighbours to walk the street with me to observe and chat about the trees on our street – both new ones and old ones.
Idea for a Research Trial (Pilot Project) on the Survival and Growth of Different Sizes of Planting Stock for Street Trees in Halifax
Note #01 by Peter Duinker, February 2022
1_Background
A small research group led by me at the School for Resource and Environmental Studies, Dalhousie University, has been collaborating with HRM Urban Forestry since 2010 on urban-forest programming in the city. First, we helped develop the HRM Urban Forest Master Plan (https://www.halifax.ca/transportation/streets-sidewalks/urban-forestry) which Council unanimously endorsed in September 2012. Since 2013, each summer our group, comprised of me and 2-4 research assistants, undertakes a range of research projects on the HRM urban forest (for a listing, see this report: “Urban Forest Management and Research in Halifax, NS”, which you can find at https://www.halifaxtreeproject.com/reports). Our main focus is collecting data on the survival, condition, and growth of newly planted street trees. The planting program during the past decade has ramped up from about a thousand trees per year to over three thousand per year.
As a researcher, I am constantly posing questions about how we do things in urban-forest management and then imagining what kind of research project we could do to shed light on specific questions. One aspect of street-tree planting about which I am inquisitive is why we plant trees that are so large at the time of planting. My scholarly and practical background is in silviculture, not arboriculture. In the woods, when we plant trees, they are seedlings. Sometimes they are potted stock no more than 20 cm tall, sometimes they are bareroot stock up to 50 cm tall. We never planted the typical balled-and-burlapped (B&B) stock that frequently is 300-400 cm tall.
Doubtless there are good reasons to plant larger stock. They are less prone to damage due to vandalism, mechanical damage from lawn-mowing equipment, and small mammals. They also give a sort of instant gratification in that, immediately, we have a tree taller than ourselves. However, they also have drawbacks. One is the time it takes for the planted tree to re-establish a healthy root-to-shoot ratio. When the tree is taken out of the ground at the nursery, the vast majority of its roots are left behind. If you tried to take all the roots, the root ball would be too large to handle. So, for several years in the new streetside environment, the planted tree allocates most of its growing resources into re-establishing a root system that matches the crown. This is highly stressful to the tree.
Another drawback in my view is the cost to plant a B&B tree. In HRM, that cost has risen from under $400 apiece about ten years ago to more like $600-700 apiece today. When seedlings are planted in the woods, they cost way less than one dollar each to plant. Part of the planting cost of large-stock trees in the streetscape is the purchase price of the tree - $200 apiece is not uncommon (and nowadays, they all come from large nurseries in Ontario because nobody grows B&B stock in Nova Scotia). The other part of the price is the labour and equipment cost. B&B stock is large and heavy so machines are needed to dig the hole and lift the tree off the truck and into the hole. When someone plants a seedling, all that is needed is a shovel.
I have heard it said that if you plant a seedling next to a B&B tree (of the same species) and they both survive in the streetscape, in ten to twenty years you would be hard-pressed to tell which was which. I would like to test that hypothesis with this study.
2_Study Design
What might a research trial look like in the streets of Halifax to inquire into the survival and growth of different sizes of planting stock? My proposal is to undertake a relatively small research trial on Lawrence St. This choice of street is based on the fact that I live on this street and therefore have immediate access to the new trees for protection, observation, and measurement. I estimate there to be about 35 plantable spots total along both sides of the entire street – this would be enough for a research trial of 24 new trees.
The physical plan would be as follows. We would plant 2 species x 4 stock sizes x 3 replicates of each (for 24 trees total). I am considering using red oak and red maple (or sugar maple). The stock sizes and rooting options would be:
- seedling, potted or bareroot
- 20-mm root-collar-diameter sapling, potted - 40-mm root-collar-diameter sapling, potted - 60-mm caliper stock, balled-and-burlapped
All the planted trees - to be installed in May/June - will be mulched to a 1-m diameter. B&B and 40-mm stock will be planted by HRM Urban Forestry staff and protected and tethered according to current HRM specifications. The other two sizes of stock would be planted by members of my research team and protected with 3 or 4 stakes (wood or T-bar) set around the seedling/sapling, perhaps 25-30 cm from the stem, for protection from lawn-mowing equipment and urinating dogs. The protection stakes should stick out of the ground by about 0.5 m. Seedlings will not be tethered, and the 20-mm stock might be, a judgement that can be made at the time of planting. Tethers would be removed after two years of service. Protection stakes would be left around the tree for probably five years or more. Specific sites for the different species and sizes would be allocated partly at random (I say “partly” because the seedlings should probably be planted in the most protected sites, such as between two large trees rather than adjacent to a driveway). Structural pruning would be undertaken by a professional with appropriate training.
All trees will be measured for dimensions, assessed for initial condition, located digitally, and photographed. Watering will be done by the Dal research team when needed during the first two years of growth (i.e. the planting year and the next one). All trees will be inspected monthly and corrections made to the stakes and tethering if warranted. Measurements and condition assessments will be made each year in September (end of the growing season, but before leaf fall). Careful records of direct and indirect costs will be made for planting and maintenance. Trees that die will be replaced as soon as possible. Detailed records will be kept on tree mortality and the likely causes.
3_Roles for Lawrence-St. Residents
My hope is that residents on our street will take enough interest in the project that they are motivated to learn more about trees in the streetscape. Residents are not able to assist in the planting of trees to be installed by HRM staff, but they may have interest to assist the Dal research team on the day that we plant the small stock.
I commit to informing residents about the project, and many topics related to urban forests, in two ways. I will put a special tab (called “Lawrence St.) on the Halifax Tree Project website (www.halifaxtreeproject.com) where information about the project will be available. Second, I offer to lead street-walks at any time when a group of residents, large or small, would like to observe the trees and discuss them with me.
4_Materials and Financial Costs
As part of the annual research partnership between my group at Dalhousie and HRM Urban Forestry, this project is a joint venture. HRM will supply all the planting stock and the installation services for the larger two sizes of stock. When supplies like mulch, stakes, and tethering wire are added in, I estimate the cost of the project to be about $3,000 (not inclusive of HRM staff time).
5_The Planting Event
The research plan calls for the larger stock to be planted first (by HRM staff). Then, the smaller trees (12 of them) will be planted in one day in late May or early June. That day will be well communicated with residents and the media. Residents and others are more than welcome to come out and observe. We will make a video of the event to be posted on our website.
6_Project Leadership
As a collaborative venture that is part of the research and monitoring partnership between my research team and HRM Urban Forestry, this project is led by senior people in the two groups. For the Dalhousie research team, I take responsibility for the project. My contact details are below. For HRM, leadership for the project is provided by Natalie Secen, Contracts Manager at HRM Urban Forestry (natalie.secen@halifax.ca).
7_Contact Information
Peter Duinker, PhD (resident of Lawrence St.)
Professor Emeritus
School for Resource and Environmental Studies, Dalhousie University peter.duinker@dal.ca www.dal.ca/faculty/management/sres/faculty-staff/our-faculty/peter-duinker.html
Co-Principal, Sylveritas Ltd.
phone: 902-229-5141
Leader, Halifax Tree Project