by Connor Youngerman and Sean Dembrosky
Introduction
Air prune raised beds (APRBs) are a built nursery container elevated above the ground, consisting of one or more stacked wooden frames with a mesh bottom filled with growing media. APRBs are an innovative and scalable approach to tree seedling propagation. This production method enhances root architecture (Miller and Bassuk 2018; Feng et al. 2021), increases transplant success (Van Sambeek et al. 2016), and allows for high-density seedling propagation (Evans and Griscom 2021) in urban and rural contexts. This factsheet (and the video below on how to build APRBs) explores the best practices, potential candidate species, research avenues, and applications for APRBs in urban forestry, agroforestry, and reforestation initiatives. We expect this information to be regularly updated as we learn more about APRBs.
Principles of Air Pruning
Air pruning occurs when root tips are exposed to air, triggering a natural pruning of exposed tap roots (i.e., tap roots stop growing), and stimulating lateral root development instead. APRB are suspended off the ground, and the base frame has a wire mesh bottom, which naturally exposes taproots to air when they grow through the growing substrate. Unlike traditional containerized systems – which can lead to root circling, crooked tap roots and poor transplant success – APRBs foster fibrous, well-structured root systems which can help seedlings establish more successfully in the field, particularly on degraded or marginal lands where root vigor is critical for survival (Van Sambeek et al. 2016).
APRBs have been shown as an excellent method for growing chestnuts (Evans and Griscom 2021), apple grafts (Elsysy and Einhorn 2022) and a suite of urban tree species (Miller and Bassuk 2018). Almost any tree or shrub species can be grown in an APRB however, and lack of published science articles should not be interpreted as lack of application. Based on our conversations with NY nursery producers, we confidently know that the following species can be grown in APRB: oaks, hickories, chestnuts, hazelnuts, pawpaws, persimmons, korean pines, mulberries, willows, witchhazel, juneberry, pears, peaches, plums, cherries. We recommend focusing on high value species with large tap roots to start such as nut-bearing trees.
APRB production is typically a seasonal practice: Seeds are sown in early spring (typically after cold stratification), and seedlings are transplanted from the APRBs in the fall as bare root stock. Due to their elevated and exposed design, APRBs leave plants vulnerable to sub-zero temperatures and wind, which can lead to significant winterkill during prolonged adverse conditions. Some species, such as white oak and hazelnut, may be robust enough to be sown in the fall and overwintered; other cold hardy species may be robust enough to be overwintered as seedlings. We are actively researching these ideas and will update as the data rolls in. If you decide to try overwintering seeds or seedling, we highly recommend that you insulate the APRBs with old straw bales or woodchips to reduce cold exposure. However, as a general rule you should plan to clear your boxes in the fall as standard practice, especially for trees that are only marginally zoned for your region.
Why APRB
Seedling production pipelines in the US are not prepared to meet the reforestation and agroforestry needs of the future. Currently, tree-planting efforts are biased toward a few core species that can be reliably propagated and readily sourced from nurseries (Broadhurst et al. 2015). Conifers make up more than 97% of the seedlings grown in the United States, with the vast majority (80%) produced in the southeastern states (Haase et al. 2021); these are rarely the species needed for conservation, reforestation, or agroforestry projects. For such projects there is a paucity of appropriate stock, and a 2.3-fold increase in tree production will be needed just to meet the broadly proposed short-term reforestation goals in the contiguous United States (Clark et al. 2023).
Complicating the production issue is the reality of the nursery workforce. Labor shortages and aging demographics are among the biggest limitations for scaling nursery capacity; recruitment and training of new workers is not keeping pace to fill the skilled-labor gap, and to prevent the loss or degradation of institutional knowledge. Unfortunately, efforts of recruitment have fallen short (Haase and Davis 2017). Learn more about CCE Tompkins County’s program to help address the nursery workforce shortage.
The tandem needs of seedling production and nursery workforce development can be addressed, in part, by APRB. This method is scalable, modular, and adaptable; with reasonable costs for establishment and low skill requirements to manage it. APRB production is suitable to rural, urban, and suburban landscapes, so allows new entrants with limited land access to participate in seedling production. In turn, this supports a decentralized seedling production network that can respond to regional reforestation needs using regional ecotypes as seed sources.
Creativity
There is no standard – industry, academic, or otherwise – to make the optimal APRB. The area, depth, building materials, and growing media can all be adjusted to the scale and sourcing that works best for you, your budget, your space, and your target tree species. This fact sheet represents a consensus among nursery professionals for APRBs, and we highly encourage folks to tweak and play with our recommendations (and let us know what you did!). With proper care, these beds can last 5 years or many more (with the right materials), making them a cost-effective investment and really worth the time and energy of building.
Siting
Most species you want to propagate in an APRB will do better in full sun (some species such as paw paw may prefer partial shade, or limited full sun exposure). Make sure you check the requirements of whichever species you plan to grow. Generally, choose a location that is flat and reasonably close to a water source for irrigation. If you are setting up in a grassy area, consider laying down cardboard or woodchips first to keep weeds from growing up around the APRB.
Key design features
- Wooden frames constructed from rot resistant materials (e.g., black locust, white oak, larch; hemlock and pines work but may not last as long); two or more stacked courses will make transplanting seedlings from the beds easier, and retains more of the growing media. We caution against using pressure treated lumber.
- Wire mesh for the bottom frame heavy gauge ½” x ½” or ½” x 1”; We highly recommend against adjusting these parameters as any larger aperture will allow rodents to get through, and any lighter gauge is more likely to rust and break.
- Woodchips or sticks as the base layer over the bottom mesh. Only a few inches needed, meant to help hold in soil media from falling through and provide a fungal resource later in the season for roots.
- Protective mesh or hardware mesh cage to prevent herbivory from rodents and deer (½” inch recommended).
Materials for the frame
A standard 4’ x 2’ APRB with three 4” deep courses requires:
- ~36 board feet of 4” wide lumber; some extra for battens if desired
- 8 square feet of ½” heavy duty galvanized mesh for base
- 48 square feet of of ½” galvanized mesh for cage
- Five concrete blocks or bricks for elevation; or three lengths of 2’ off cuts of wood to span under box
- Exterior grade screws and heavy duty staples
Tools
- Drill (we recommend predrilling your holes, especially if using hardwood like locust)
- Impact driver
- Saw (we use a table or circular saw, but a sharp handsaw will work just fine)
- Square
- Tape measure and pencil
- Safety gear
Growing media
Formulating your growing media is the most important stage of creating an APRB. The goal is to have a well-draining yet moisture retentive media with good aeration and high nutrient availability (but not too high). Build the media up in layers for best results; make sure your APRB is sited and the frame is set on blocks before adding media. We do the following, but it’s fine to modify based on your budget and materials on hand:
- Bottom layer: coarse woodchips or sticks layered 1-2” deep (aged woodchips are best)
- Middle layer: well mixed combination of 3 parts aged compost, 1 part perlite, 1 part aged woodchips or sawdust; spent mushroom substrate or wine cap spawn is great addition too
- Top layer: thin layer of middle layer mix to cover seed and fine wood chips or sawdust as a liberal mulch after planting.
Water
Ensuring steady, appropriate moisture is one of the most important factors for successful propagation in APRBs. APRBs dry out faster than in-ground plantings because they are elevated and exposed to air on multiple sides, so they need more frequent watering. If the bed is allowed to dry out completely, seedlings can be seriously stressed or killed, so plan for regular checks and irrigation. Beds sited in hot, open, or paved areas (such as parking lots or open fields) will usually need more frequent watering than beds in a sheltered yard or partially shaded spot. Mulching around and below the APRB with hay/straw/woodchips can help keep the microclimate cooler and reduce bed dessication. If you growing on exposed pavement, you may also consider adding shade cloth over the APRB. Water often enough that the growing media feels slightly damp when you press a finger below the top surface, rather than dry or soggy. In many seasons, regular rainfall may be enough, but you should supplement with a hose or watering can whenever the media starts to dry out. If you are managing several APRBs or cannot check them daily, consider a timed drip system.
Weeds and other pests
Weed control is essential until seedling canopy closure. If seeds are planted densely, canopy closure occurs quickly. Top dressing the APRB with light mulch will also help keep weeds down. For the first couple months, check each APRB every 1-2 weeks and remove any weeds. Weeding should not be necessary later in the summer.
Rodent protection is critical for APRBs, as small mammals such as voles, mice and squirrels can damage seeds and young seedlings. If you build a two or three-course APRB ensure that the gap between courses is less than ¼” to prevent rodent access. If there is a gap, you can cover it with hardware cloth, wood, or sheet metal.
It is highly recommended to build a hardware cloth cage to put over the APRB so rodents cannot get in from the top, and deer cannot browse on young seedlings. If you are in a heavy deer population area, consider electric fencing around the APRB.
Harvest and Storage
Air-pruned raised beds are designed for one growing season of seedling production and are not intended for overwintering seeds or seedlings in place. Plan to lift seedlings in fall once leaves have senesced or have been stripped, while plants are fully dormant but before deep freezing winter temperatures. In New York and surrounding regions, harvest typically begins mid-October and wraps by mid-November.
To harvest, remove the cage and top course so you can reach roots from the side, then gently lift individual seedlings or small clumps by hand. Lightly tease root systems apart and return as much loose media as possible to the bed or into a nearby bucket for reuse. Bare-root seedlings should be bundled right away; wrap the roots in wet burlap or other moist material and keep bundles shaded and out of direct sun to prevent desiccation. For storage beyond a brief holding period, “heel in” seedlings by temporarily planting the roots in loose, moist soil in a cool, shaded spot until you can move them to their permanent site. Whenever possible, plant seedlings into their forever home soon after they come out of the APRB to reduce transplant stress and mortality.
Empty APRBs can remain outside over winter with the growing media in place, recognizing that elevated beds expose media and any remaining roots to deeper freezing than in-ground plantings. Where lumber is not naturally rot resistant, beds can be disassembled and stored under cover to extend their life. Growing media can be stored and reused in the next season, but it should be refreshed with new compost, mineral soil, or other amendments to replace nutrients and organic matter that were depleted during the previous crop. Whether or not you empty the courses of media for the winter is larger about the type of wood you used to build: if courses are made of rot resistant wood, like black locust or oak, they can be left full for the winter.
Candidate Tree Species for APRB Propagation
Most of the (scant) literature available on APRB propagation focuses on nut-bearing trees, or other species with long tap roots. Anecdotally, growers report success using this method for conifers, cuttings, urban-adapted species, and most fruits. We think most trees and shrubs can be grown using APRBs, and are always interested to hear about peoples’ experiences.
Conclusion
Air prune raised beds offer a promising alternative to conventional tree propagation, with wide-ranging benefits for urban forestry, agroforestry, and ecological restoration. Continued research and adoption of this technique can expand tree planting initiatives, increase reforestation success, and support more resilient landscapes. By integrating APRBs into broader conservation and agricultural strategies, practitioners can cultivate trees more efficiently while advancing sustainability goals.
References
Clark, P. W. et al. A lack of ecological diversity in forest nurseries limits the achievement of tree-planting objectives in response to global change. BioScience 73, 575–586 (2023).
Elsysy, M. & Einhorn, T. C. Air-Pruning Containers Modify Root and Scion Growth and Alter Resource Allocation of Bench-Grafted Apple Plants. Horticulturae 8, 797 (2022).
Evans, T. & Griscom, H. Comparing the effects of four propagation methods on hybrid chestnut seedling quality. Trees, Forests and People 6, 100157 (2021).Fargione, J. et al. Challenges to the Reforestation Pipeline in the United States. Front. For. Glob. Change 4, (2021).
Feng, Z. et al. Improvements in the Root Morphology, Physiology, and Anatomy of Platycladus orientalis Seedlings from Air-root Pruning. horts 53, 1750–1756 (2018).
Haase, D. L. & Davis, A. S. Developing and supporting quality nursery facilities and staff are necessary to meet global forest and landscape restoration needs. REFORESTA 69–93 (2017) doi:10.21750/REFOR.4.06.45.
Haase, D. L. et al. Forest Nursery Seedling Production in the United States—Fiscal Year 202. 65, (2022).
Levinsson, A. Post-transplant Shoot Growth of Trees From Five Different Production Methods is Affected by Site and Species. isa 39, 201–210 (2013).
Maguire, T. & Harun, R. Air Root Pruning to Accelerate the Growth of Elaeagnus ebbingei From Vegetative Cuttings©. 57, (2007).
Miller, B. & Bassuk, N. The Effects of Air-Root Pruning on Seedlings of Species with Taproots. Combined Proceedings IPPS 68:145-151. 2018.
Van Sambeek, J. W., Godsey, L. D., Walter, W. D., Garrett, H. E. & Dwyer, J. P. Field Performance of Quercus bicolor; Established as Repeatedly Air-Root-Pruned Container and Bareroot Planting Stock. OJF 06, 163–176 (2016).
