Effect of Container Depth on Taprooted Seedling Root Morphology and Post-Transplant Establishment Success

Progress report for FNE20-947

Project Type: Farmer
Funds awarded in 2020: $14,908.00
Projected End Date: 02/28/2022
Grant Recipient: Full Fork Farm
Region: Northeast
State: Maine
Project Leader:
Anson Biller
Full Fork Farm
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Project Information

Summary:

Taprooting perennials, by their nature, developed a survival strategy of sending a single primary root deep into the soil before shifting focus to robust lateral root growth. Nursery production of these perennials, by its inherent constraints, must artificially arrest taproot growth to produce a plantable sapling. Not doing so results in root-binding that is far worse for trees post-transplant success.

While concerted research has investigated the effect taproot pruning has on root morphology within containers, limited research has explored the effect that container depth and, by extension, taproot length/development has on the post-transplant establishment of saplings.

What is the ideal container depth that grows the highest quality nursery stock and encourages best initial vigor and survival? Is this the same across applications, whether agricultural or reforestation? The project is designed to investigate these questions using chestnuts grown from seed in containers of various depths, collecting data of initial seed weight, followed by survivorship, dry weight, taproot diameter at 4", taproot length, and caliper. Saplings will be transplanted to a test plot for a year of observation. Survivorship, tree height, caliper, and diameter at breast height will be recorded at the end of the year.

I will seek advisement primarily from Aaron Parker, a local nursery operator, and will also draw on the consultation of Caleb Goossens, MOFGA's Organic Crop & Conservation Specialist. Outreach will be conducted through written articles, online posts, and conference/event presentations.

Project Objectives:

This project seeks to determine the effects that nursery container depth has on taprooted seedling development in air-pruned nursery pots, and how these differences translate to post-transplant survivorship and initial vigor under contrasting management strategies that simulate real-life application.

In the first year, we will grow a total of 625 chestnuts from seed in air-pruned containers of equal diameter within five different soil depths (6", 9", 12", 15", and 30" - 125 seeds each). The collected results will provide nursery growers guidance on optimizing container production for taprooted seedlings.

In the second year, a random sample of 40 trees from each nursery group will be planted at 20' in-row spacing (32' between-row spacing) under the two contrasting management strategies detailed earlier in this application, for a total of ten experimental groups, and post-establishment data will be collected for a season. This aspect of the project seeks to draw out if/which root morphologies are better suited for particular applications (ex., agricultural vs. ecological restoration/forestry). Solid data on this will help inform purchasing decisions of nursery stock and propagation practices to improve project success rates.

Introduction:

Healthy plant stock is critical for post-transplant success and profitability in business ventures. The challenges of producing healthy root systems in nursery containers are well-documented and are of great importance to nursery growers, restoration ecologists, foresters, and farmers. This is especially true of taprooting perennials: "Tap-rooted plant species are particularly vulnerable to becoming root bound because they initially develop a central leader or taproot during germination, which only later develops lateral roots," (Burkhart 2006). Arizona Cooperative Extension agent Rick Gibson explains that  "the taproot is a critical storage organ for water and food supplies," which "quickly finds water and nutrients" when a seed first germinates, and is a reserve during challenging drought years (Gibson 2019). Burkhart, in his paper, goes on to relate research projects that found California chapparal and oak woodland species root systems typically range from 7 to more than 16 feet deep in nature. It being the case that the primary root plays this important role, what are the near- and long-term effects of taproot pruning on saplings transplanted into the field? How should it inform the nursery best practices, and purchasing decisions of tree stock? This project seeks to focus on the near-term implications of this question.

One thing to acknowledge is that containerized nursery production will always be an unnatural context for a seedling. It is a human endeavor with inherent constraints. At the same time, we as humans are often planting them into unnatural contexts. A farmers' irrigation, fertilization, staking, and cultivation decisions both have intentional and unintentional consequences to what happens within the soil profile. A plant's root system responds to take advantage of these cultural practices. I hypothesize that the ideal root system depends on the context trees are planted into. Provide a tree intensive care from above as one might see in an agricultural context, and a robust, initial system of lateral feeder roots could provide an edge. On the other hand, if one's intention is to plant a sapling and step away - as might be the case in an ecological restoration or forestry project - planting a tree with an intact taproot may hold the key to its survival and success.

In practice, the solution is unlikely to be black and white. This is an aspect of what this study seeks to investigate. Is there a container depth that optimizes balance between depth for taproot growth and lateral root production? The results may have important implications for the ideal nursery stock with a view towards its planting context. How do the root morphologies we encourage in the nursery play out under either intensive and non-intensive management?

I will test this by germinating 625 chestnut seeds using air-pruning containers with the experimental variable being container depth (6", 9", 12", 15", and 30"; 125 seeds per experimental group). Data will be collected on initial seed weight, followed by survivorship, dry weight, taproot length, and caliper at dormancy, as well as photographic documentation. Saplings will then be heeled in for the winter.

In the second season, a random selection of 40 saplings from each experimental group will be transplanted at 20' spacing into the field under two different management strategies: intensive & non-intensive. The intensive group (5 groups; 200 trees total) will receive appropriate irrigation, fertilization, mycorrhizal inoculant, staking, a tree tube for animal protection, and weed suppression using a 3'x3' weed mat that one might expect in an agricultural setting. The non-intensive group (5 groups; 200 trees total) will only be provided the tree tube and mycorrhizal inoculant. At the end of the season, survivorship, tree height, diameter at breast height (DBH) will be measured for all groups.

This experiment is designed to determine the optimum container depth for nursery production of chestnuts, and to hopefully provide guidance that extends to other taprooted tree species. Further, placing the chestnuts under the two management regimens detailed above may indicate whether seedling root morphology contributes to post-transplant success for particular applications. This data would better inform both propagation best practices and purchasing decisions for taprooted trees, thereby improving crop productivity by reducing tree replacement and labor costs. Ultimately, we as humans want to provide the optimum conditions for nursery plant growth to produce a transplant that thrives. My hope is that this project will contribute useful data toward this end.

Cooperators

Click linked name(s) to expand
  • Caleb Goossen
  • Aaron Parker

Research

Materials and methods:

2020 progress

Project Status: The project is successfully underway. We took initial data and seeded the chestnut seedlings in the spring of last year, growing them out until autumn. We vastly simplified irrigation by setting up an overhead system for watering. One issue encountered was that the identifying labels fading due to UV exposure. I took steps to purchase aluminum tags to re-label all the trees. In November, the chestnuts were removed from their growing containers and stored them in the farm's walk-in cooler for winter storage. Due to Covid-19, I did not pursue any in-person outreach engagements, but posted several social posts to our farm's social media and to farmer forums I participate in online.

Next steps: An impending storm necessitated prioritizing the removal and storage of the seedlings without taking final measurements. The idea was/is to complete data collection over these winter months indoors. I also still need to submit an updated budget, which I recognize is months late. Between the busy farm season, shifting needs in response to Covid-19, and a personal loss in my family, I have not been able to get to it. The project is certainly within budget, however, and I will submit the updated budget this month.

We are looking good to proceed in 2021 as soon as the soil warms enough to plant the chestnut trees. I trialed a neighbor's post hole digger this last season for planting fruit trees with success, so know that it will be a viable means of getting the chestnuts in the ground.

I will evaluate outreach methods for the project and communicate with the office regarding them in April/May.

Proposed Materials and Methods

NURSERY (Year 1):

  • 625 Chestnuts will be germinated within our greenhouse in air-pruning containers 6.5" in width and of five depths: 6", 9", 12", 15", and 30" at 125 seeds per group. Each container will be given an ID and label. Initial seed weight will be recorded.
    • Fully-ventilated ShellT grow tubes will be used as air-pruning containers for their modularity in increasing depth while maintaining a constant container width.
    • A 6.5" width has been chosen as the pot diameter based on Dr. Carl Whitcomb Ph. D.'s work, who developed the first commercially available air-pruning container. Dr. Whitcomb observed that lateral root promotion tends to only occur ~4" back from an air-pruned root tip (Whitcomb 2003).
    • 6" is the minimum container size selected in order to reflect the typical depth of a 1-gallon pot.
    • A 9" container size reflects the typical depth of commercially available deep pots.
    • The choice of a 30" container depth for the maximum size is to ensure uninhibited taproot growth.
    • Each group will be provided with a season's worth of fertility at planting, a treatment of mycorrhizal inoculant, and three treatments of compost tea during the nursery season. They will be given a similar weekly water regimen throughout the season. Type and quantity of amendments used will be noted.
    • Grow tubes will be secured in gridded, 40"x48" wooden stands with wire meshed bottoms for bottom air-pruning built on shipping pallets. I calculate needing to build 22 of these stands for the project, fitting 28 tubes per pallet.
  • Aaron and I will have monthly check-ins on the project's progress. He will be present for the first day of fall data collection, and for analysis review at the end of the season.
  • At the end of the season in dormancy, the grow tubes will be opened, and soil washed off. Data collection of each seedling will include: survivorship, dry weight, caliper, seedling height, taproot diameter at 4", and total taproot length. Photographs will be taken of each seedling against a 1"-gridded background.
  • Seedlings will be heeled in for the winter.

FIELD (Year 1):

  • A soil test of the field plot will be taken and sent to Logan Labs in Iowa. Any glaring nutrient deficiencies will be corrected based on soil test recommendations and recorded.
  • A test hole will be dug with a hired excavator to determine potentially restrictive layers like hardpan or shallow bedrock that might affect the experiment. Significant hardpan, if found, will be addressed with subsoiling for the intensively-managed chestnuts.
  • A 3'x3" weed mat will be installed at each future transplant site to passively eliminate sod.

FIELD (Year 2):

  • A randomized sample of 40 trees from each nursery group will be planted in the spring at 20' in-row spacing, 32' between-row spacing under two management strategies - intensive and non-intensive - for a total of 400 trees in 10 different experimental groups.
  • I will rent a post hole digger with a 12-24" auger from Eagle Rental in Waterville, ME for transplantation.
  • Intensively managed plantings meant to simulate an agricultural context will receive: supplemental irrigation at 1-gallon of water per tree per week, staking, a 1/8" application of compost ~15" out around the tree, a tree tube for animal protection, a 3'x3' weed suppression mat, a treatment of mycorrhizal fungi, and an 1# of Azomite rock dust or equivalent. (200 trees total selected randomly, 40 from each experimental nursery group)
    • The weed mat used for the intensively-managed groups will be the one applied in Year 1 field preparation.
  • Non-intensive plantings will only receive a treatment of mycorrhizal fungi, and the 5' tree tube for animal protection. They will receive no supplemental irrigation, staking, compost, weed mat, or rock dust. This mimics care that one might expect in a reforestation context. (200 trees total selected randomly, 40 from each experimental nursery group)
  • The in-field protecting tubes for all trees will be the same ShellT tubes used in Year 1 for seedling propagation, but snapped together to a combined height of 5' (requiring 4 ShellT grow tubes per tree). The reason the non-intensively managed trees also receive this treatment is to avoid animal damage that might compromise the experimental data.
  • Aaron and I will have monthly check-ins on the project's progress. He will be present for the first day of fall data collection, and for analysis review at the end of the season.
  • The following data will be collected at the end of the season for all groups: survivorship, tree height, diameter at breast height (4.5'), and caliper.
Participation Summary
Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.