Green Venture began planting mini forests in Hamilton in 2021. Over the past few years, we have learned a great deal about the why, what, and how of mini forests and how to support their success.
Why Mini Forests?
The hallmarks of the mini forest planting technique include: soil preparation with deep digging and the addition of compost and other amendments; dense planting with 4 trees per meter squared (m2); the use of a wide variety of locally native trees and shrub species (e.g., 15-30 species) that are planted with a multi-layered design to mimic the complexity and canopy layers of a native forest; and planting all this in a very small area, ~ as small as 100 m2, or about the size of a tennis court.
In contrast, most restoration and urban tree planting efforts that have occurred in southern Ontario do not include soil amendment and improvement, trees are spaced at least 1 meter apart, and generally only a few species of locally native trees or shrubs would be planted. Tree planting in these urban sites faces several challenges, including limited available space, degraded soils, a lack of moisture retention, the need for external watering, as well as challenges with herbivory, competition from annual weeds, a lack of aftercare, and stewardship.
The mini forest approach aims to establish small but diverse and ecologically impactful forest patches in densely populated urban areas. Practitioners and communities adopting mini forest planting techniques report that trees grow twice the speed normally expected, weed competition is suppressed, soils are enriched and retain more moisture, and that mini forest patches support greater biodiversity.
Why Mini Forests in Hamilton?
Since 2021, we’ve been piloting mini forests as a method to increase canopy cover and grow Hamilton’s urban forest. We wanted to experiment with the mini forest method for Hamilton because we desperately need to increase canopy cover in urban areas and improve tree equity, and because we noticed that past community tree planting efforts have experienced limited success (ie. tree mortality due to herbivory, mechanical damage from lawn mowers and whipper snippers, and a lack of watering).
Embracing the Miyawaki Method, developed by Japanese botanist Akira Miyawaki, our projects include:
- 10 – 100m2 community mini forest plantings at Windermere Basin Park (two mini forests installed here in 2021 and three more mini forests and a fourth control site installed in 2023), and at sites like Lake Avenue Park, Morton Park, and Mohawk College.
- 8 – 30m2 ‘Mini Forests at Home’ were planted at EcoHouse in 2021 and by residents and schools in 2023. Learn more about our Mini Forest at Home kits and resources on our webpage.
As part of the Green Communities Canada and Network of Nature National Mini Forest Pilot project, the City of Hamilton Forestry section also planted a mini forest at Johnson Tews Park during the fall of 2023. This pilot project aims to contribute to Canada’s goal to plant 2 Billion Trees by 2030 to address climate change and biodiversity loss.
As we are coming up on our third post-planting year, our mini forest sites are demonstrating that mini forests can be an important tool in our tree planting toolbox for several reasons. Notably, our mini forest plantings are growing extremely well, with a 30 cm average annual growth increment and an average survival rate of 95%.
Our sites demonstrate minimal weed growth (e.g., annual and perennial non-native grasses and forbs), have received no external watering post-planting, and we’ve begun to document soil improvements, including in bulk density, soil organic matter, particulate organic matter, and pH.
Lessons Learned:
Adapting the Miyawaki Mini Forest Method to our Hamilton, Ontario, soils and Mixedwood Plains Ecozone has taught us many lessons. We’ve learned that site selection, building the soil, native plant sourcing and selection, and stewardship and monitoring are key factors in ensuring successful mini forest outcomes.
Choose Sites Wisely
Wise site selection has been an important factor in developing mini forests. We suggest assessing and securing multiple potential locations to add a mini forest. Sites can be in a neighborhood near you, an area lacking in tree cover or tree species diversity (e.g., equity deserving sites), or a park that you love to visit and would like to see increased biodiversity. Choose a site that has partial to full sun for a mini forest to thrive. Mini forests can be planted as an extension of an existing forest or in open areas and fields to create a new forest ecosystem. Ensure these sites are publicly accessible for volunteers, plant delivery, and watering trucks.
Build Your Soil – Site Preparation
The traditional Miyawaki Forest Method prescribes tilling and deep digging down to a meter depth to amend the soil. In all our mini forest plantings, we did not till or remove any of the existing soil. Hamilton soil consists primarily of Grey Brown Luvisol, originating from limestone bedrock and glacial moraine deposits. Under a healthy forest canopy, the mixing of leaf litter and other organic matter by soil organisms would have resulted in a unique layer of organic material – known as a ‘mull Ah horizon’ in soil classification. Underlying this thin layer of organic material are distinct layers (A, B, and C horizons), which evolved over time with differing clay contents and calcium carbonate levels originating from the parent bedrock materials. Deep digging into these lower horizons with higher clay content would cause significant soil disturbance and result in damaging any existing organic topsoil and bringing heavier clay soils into the higher soil horizons.
Our site preparation method focused instead on amending and regenerating the soil by adding organic materials on top of the topsoil to enhance the existing organic material layer and build the soil food web. Our soil preparation approach has been: 1) adding a layer of cardboard (to suppress existing grasses and annual weeds growing on the site), 2) followed by 1-2 inches of compost (e.g., manure loam), and finally, 3) adding 2-4 inches of mulch (e.g., arborist chips or cedar mulch) on top of the turf where the mini forest was to be planted. This approach results in a 4-7 inch layer of amended soil at each site.
We aimed to have soil amendments completed at least a season before planting occurred. For example, we would complete site preparation in the spring for a fall planting. At one site, mulch was placed directly on topsoil, and during planting, we added some compost to the planting holes. At all the sites where soil amendments were placed, we are seeing low weed growth, and no sites have received external watering after their initial planting. We will continue to monitor soil conditions to better understand their growth patterns and benefits compared to conventional tree planting site preparation.
Choosing and Sourcing Native Species
We source a wide variety of locally native species, selecting species found in the 4 canopy layers (e.g., ground cover, shrub, understory,sub-canopy and canopy species). We avoided pioneer species such as poplar and willow species, as mini forests are made up of mid- to late-succession species, and pioneer species are the first to colonize newly created environments. Securing locally native and seed source identified trees and shrubs may be a challenge for some areas. We suggest building a species list by conducting research prior to planting on what types of species align with your environmental conditions and native ecosystems. For example, you can investigate local forests to identify species thriving in the area or consult with experts such as arborists, ecologists, master gardeners, and traditional knowledge keepers. In Ontario, lists of native tree and shrub species by ecodistrict are available from the Forest Gene Conservation Organization. For all of our plantings, we selected species found in our local ‘Red Maple – Sassafras Deciduous Forest’, ‘Hemlock – Sugar Maple Mixed Forest’, or ‘Hickory-Maple-Oak Deciduous Forest’ forest types and consulted with the Network of Nature and the Mini Forest Pilot Program resources for species recommendations and quantities. Finding locally native seed sources may also be a challenge encountered in some areas, especially for scaling up mini forests. We suggest identifying local native plant nurseries that follow seed certification standards in your region. For example, Ontario Natural Selections or the Ontario Native Plant Growers Association.
Stewardship is key!
Caring for native plant species in urban environments presents various challenges, including competition from non-native or invasive plants, human-induced damage, and tree stress due to drought. Monitoring and maintaining your newly planted mini forest is crucial to supporting long term growth. Green Venture is dedicated to stewarding our past mini forest sites! With the help of volunteers, we tend to these plantings 1-2 times a year during the growing season by removing and disposing of any weeds or invasive plants, and cleaning up any garbage around the area.
Evaluating Mini Forest Outcomes
Last year, Green Venture announced a $35,000 grant from the Hamilton Community Foundation’s Community Health, Education, and Research Fund to evaluate Hamilton’s first mini forest plantings and future plantings. As part of this evaluation project, we planted three test mini forest plots and one conventional tree planting at Windermere Basin Park in November 2023.
With the help of the Vineland Research & Innovation Centre, we are assessing several soil health measures under various treatments: 1) the October 2021 mini forest at Windermere Basin Park; 2) the October 2021 conventional tree planting site at Windermere Basin Park; 3) the baseline soil (collected prior to the new mini forest and conventional tree plantings in October 2023). The Mini Forest in Hamilton Project involves implementing the Mini Forest planting method in a designated area and monitoring it alongside an established Mini Forest for a minimum of five years. Both the recently planted mini forest (Nov. 2023) and the established mini forest (Oct. 2021) are located next to a conventional tree planting area, providing a comparative context for assessing soil health and plant growth impacts. On October 4th, 2023, the Vineland team collected 18 soil samples from these areas, with three sampling points in each, capturing both topsoil and subsoil layers at depths of 0-20 cm and 20-40 cm, respectively. We will continue to monitor this site to compare and contrast the difference in soil health between conventional tree planting methods and mini forest plantings.
How can you help?
To get involved in this project as a volunteer, keep on the lookout for our mini forest planting happening this fall or help us steward our past mini forests from May to September through our Upcoming Events webpage.
More information:
🔗Vineland Research and Innovation Centre
🔗Nature of Network: Mini Forest
🔗Green Communities Canada: National Mini Forest Pilot Project
🔗What is the Miyawaki Method of Afforestation
The Hamilton Mini Forest Project’s work is funded through the Hamilton Community Foundation, with support from the Conserver Society of Hamilton.
