Guide to Assembling a Large Horticultural Container for a Biologically-Intensive Horticulture

Dr. Richard Freeman

Author’s Note

This guide is in draft form. It includes no foot notes or references and it contains significant omissions. Anyone interested in republishing using the content in this guide should first contact me, the author, Richard Freeman, to gain permission.

I am interested in co-authoring a final version of this guide.

The information I present is for educational purposes and I am not liable or responsible for anyone’s use or misuse of this information.

I will periodically post updates to this blog by adding more detail.


Urban horticulture is especially novel because it is so spatially constrained. The grower must produce a good return from a small space, depending on inputs from the surrounding environment. Thus, getting the most out of the working space if imperative. Sustainable container horticulture is further concerned with the sustainability of the substrates and inputs it uses.

This guide contributes to the concerns of developing sustainable horticulture container media (HCM), optimizing space and conserving and recycling resources by providing a unique framework for assembling a large horticultural container. The framework uses multiple substrates with contrasting properties in a container zoning system (aka spikes and layers) to combine maximum nutrient loading and a robust biology with the physical properties of an aerated mix.* This guide offers a framework specifically for assembling a container to maximize fertilization aimed at high production vegetation and inflorescence in large containers (7-15 gallons).

Adopting the system will require a cycle of live testing and sampling and periodic data collection to determine precise watering needs in the context of the specific plants, HCM, containers, environments and management styles. This guide explains the basic procedures in general terms. To note, the container system this guide describes is best suited for an on-demand irrigation system calibrated to maintain readily available water without leaching nutrients and draining water.

This guide contains four sections. Section 1. describes the components of the zonal system. Section 2. briefly describes the process of filling a large container using the container zonal system. Section 3. describes the process of determining weight thresholds for triggering irrigation and for determining irrigation volumes.

Section 1. Overview of the large container zonal system

From a functional level, horticultural containers tend towards being spatially constrained, nutrient-constrained, lacking robust buffering capabilities and tending towards poor aeration and low volumes of readily-available water. In contrast, a high-performance container ecosystem depends upon a structure that accommodates intensive nutritional and biological loading while providing optimal aeration and readily available water.

1.1. Zonal system components

The effective container volume of any container will be equal to the full container size minus the volume of the watering space. For an example, I use a 10-gallon bag (with handles), with an effective container volume of 9.3 gallons (watering space 1 inch down from the top).

The container zoning system segregates HCMs with contrasting properties, creating three types of zone within the effective container volume. Zone A has high aeration, abundant readily available water and relatively high nutrient and biology loading. Zone B has maximum aeration and drainage, minimal available water and high nutrient loading. Zone C has low-aeration, minimal available water and extremely high biological and nutrient loading.

1.1.1. Zone A

Zone A occupies the largest share of container space with a high-aeration HCM mix (henceforth HCM-A) that optimizes aeration and readily available water (RAW). For this draft, RAW includes the volume of water available at tensions of 1 kPa to 16 kPa. This range includes easily available water, available at 1-5 kPa, water buffering capacity, available at 5-10 kPa, and a reserve between 10-16 kPa. This latter reserve is part of a reserve between 0.8-16 kPa empirically determined to maintain plant production equal to plants growing within the water buffering capacity.

Roots occupy Zone A first with fairly even distribution (in a fabric grow bag). In Zone A, nutrient loading will promote maximum growth in the vegetative stage. This zone occupies roughly 80% of the effective container volume.

In the reference example, it requires 7.5 gallons of aerated HCM. The preferred HCM for Zone A meets the needs for maximum aeration, abundant readily available water and relatively high nutrient and biology loading. This HCM, by design, contains the maximum quantity of nutrients and microbes that a plant in vegetative stage could tolerate without symptoms of nutrient overloading such as salt burn.

1.1.2. Zone B

Zone B occupies the bottom of the container to a depth of 2-3” with a simple HCM mix including a porous coarse substrate loaded with a combination of nutrients intended to maximize bloom and flower production (HCM-B). Roots tend to occupy this zone towards the end of the vegetative cycle. This zone occupies roughly 15% of the effective container volume, or 1.7 gallons of the coarse substrate in the reference example.

The preferred HCM for zone B meets the needs for high aeration, minimal available water and extremely high nutrient loading It constitutes the bottom layer of the container. In the reference example, HCM-B occupies the bottom 2.5 inches of the container.

In regard to physical properties, zone B provides air passage from the sides of container to the center, so particles must be large enough to provide abundant inter-particle air space that is continuous. The volume of air space offsets the high tortuosity of large-particle media, off-setting effects on diffusion to and from the rhizosphere.

In regard to chemical and biological properties, nutrients that are either available to the plant or to the rhizosphere ecology occupy an extremely high percentage of HCM-B, with an emphasis on plant needs during blooming.

In the reference example, HCM-B is a coarse, infused biochar with 90%+ larger than 3/8-inch diameter and 100% smaller than 3/4-inch diameter. The biochar is infused with a high-nitrogen bokashi exudate, which contributes modest nitrogen, since overall volume of exudate exposed to the rhizosphere is minimal. In addition, the biochar is adsorbed with a mixture of salts suited for high-production blooming. This HCM occupies1.75 gallons in the large container.

This zone holds no appreciable amount of available water and will not lose significant weight as the container dries to wilting point.

1.1.3. Zone C

Zone C is distributed evenly through Zone A in the form of vertical spikes and pellets (HCM-C) and will contribute to vegetation and flower demands with intensive nutritional and biological loading. Roots tend to occupy this zone, which is distributed amid the HCM in Zone 1, starting by surrounding the outer surface of the pellets or spikes around the time of anthesis. Throughout flowering the roots penetrate the spikes and layers. This zone occupies roughly 4.7% of the effective container volume – 0.6 gallons of dense substrate in the reference example.

The preferred HCM for zone C meets the needs for low-aeration, minimal available water and extremely high biological and nutrient loading. This zone is made of vertical spikes.

In reference to physical properties, zone C is highly dense with small particles. It provides minimal air diffusion and modest available water.

In reference to chemical and biological properties, this HCM is highly concentrated in biological mass and infused with nutrient salts to promote late vegetative growth and flowering.

An example HCM-C blend could include fine particles of green waste compost, vermicompost and raw biochar at ratios of 1:7:8, respectively with a blend of nutrient salts and vegetation-based nitrogen. Over 90% of particles will be less than 1.0 mm.

This zone also will hold no appreciable amount of available water or air-filled space, and it will not lose significant weight as the container dries to wilting point.

1.2. Zones create Edges

In addition to creating contrasting functional zones, the zoning structure creates micro-edges between contrasting substrates, encouraging aerobic, microbial diversity. These edges present extreme gradients that fluctuate with transition between gas and liquid phase in the soil water.

Likewise, horizontal edges, especially between Zones A and B, can create conditions for perched water-tables, though the capillary rise is mitigated by the uneven horizontal edge between the HCMs and by the strong water-holding abilities of the bottom layer.

Section 2. Assembling the container environment

Assembling the container environment is straightforward. The following steps outline the process.

2.1. Moisten HCMs A and B if not already moist.

2.2. Place a layer of HCM-B at the bottom of the container 2-3” deep.

2.3a. If using HCM-C pellets, mix them evenly with HCM-A.

2.3.b. If using spikes, loosely place sections of plastic plumbing pipe of chosen diameter and length in the corners of the container. In the reference example, spikes are 2 inches in diameter and 10 inches long.

2.4. Place enough HCM-A or HCM-A and B mix to occupy a 2-inch layer atop the layer of HCM-B. If using spikes, position them so they will remain upright in the corners.

2.5. Place an empty container the size of the transplant containers (in the reference example, a #1 pot) in the middle of the bag atop the layer of HCM-A.

2.6. Pour most of the remaining HCM-A or A-B mix into the container.

2.7. Shake the bag up and down to settle HCMs.

2.8. Pour remaining HCM-A or A-B mix into the container.

2.9. If using spikes, pour HCM-C into the spikes, tamping every cup. Slowly pull the pipe after filling the cavity, pushing the HCM-C through it bit at a time.

2.10. Moisten evenly up to one-tenth container capacity.

2.11. Remove empty transplant pot and plant an annual-season, high-producing plant.

Section 3. Irrigating the big-container system

This system of assembling a large horticultural container with a maximum level of available nutrients requires an irrigation strategy that will minimize nutrient leaching, while always maintaining a reserve of readily available water. The HCM in this system are extremely nutrient-rich. Consequently, over-watering will cause significant leaching and can create chronic anaerobic conditions, leading to root rot, pest nematodes and other problems. Likewise, under-watering causes plant stress and poor re-wetting efficiency. Thus, the grower must find a cost-effective balance between these extremes that will also accommodate efficient irrigation scheduling. The optimal irrigation regime will maintain continuous access to evenly-distributed RAW during photosynthesis and will leach no media solution.

A simple gravimetric system for determining irrigation needs, based on the weight of the container and plant, is a low-cost sensing system well-suited to the zonal system because it conserves water and minimizes leaching. When the container has reached a certain weight (or a weight within a range of values), the person or automated system applies water to increase volume to container capacity. This weight or range of weights corresponds to a water volume below the threshold of readily available water but above the initial wilting point.

3.1. Simplified gravimetric system for small horticultural operations

A small horticultural operation can easily get by without correlating water availability to container weight or with using sub-statistical samples. This subsection describes a procedure for establishing a simple gravimetric sensing system to determine when plants need irrigation and how much water per plant to irrigate.

This procedure requires determining container weight (weight of a full container) and water volume at container capacity and at initial wilting point and. Determine container weight requires determining the volume of water the container holds at each stage (level) of water availability.

3.1.1. Determining container weight at container capacity

Follow these steps to determine weight of the full container at carrying capacity: Weigh the assembled container and record the weight to one decimal place (xx.x lbs). Place the assembled container in a larger, empty container that will hold water. Water the container until the media is saturated. Consistently wet the entire surface until water is running out the bottom and into the larger container. Allow the water to run until the larger container is half full. Let the container sit for 10 minutes then remove it to a place where it can freely drain. Drain the container for one hour. Weigh the container at capacity and record the weight. To calculate the weight of the water at container capacity, subtract the the weight of the dry container from the weight at container capacity:

weight of water volume at container capacity = container weight at capacity – dry container weight To calculate water volume at container capacity, divide weight of volume by 8.33 (weight of a gallon of water):

water volume at container capacity = weight of water volume at capacity ÷ 8.33

3.1.2. Determining container weight at initial wilting point

The grower must establish the weight of the container and volume of water at wilting point to establish a minimal water level for determining irrigation needs and frequency.

To determine container weight and volume of water at initial wilting point follow these steps. Bring a container with a live plant to container capacity by saturating and draining (per above instructions). Allow the container to dry until the plant begins to wilt slightly. Important note: if the temperatures are above 70° F, place the plant in dappled light. Do not let the plant wilt beyond initial wilt. Weigh the container and record container weight at initial wilting. To obtain the volume of water in the medium at initial wilting, divide the container weight by 8.33 (lbs./gallon).

However, to avoid plant water stress, the minimal water level should be above initial wilting point. For this example I will set the minimal water level to roughly one third the volume increment between container capacity and wilting point.

3.1.3. Determining the volume of irrigation water

Irrigation management must provide abundant water without drowning plant roots. On one hand, the manager must provide adequate easily available water, which extends beyond readily available water (1-5 kPa) and water buffering capacity (5-10 kPa) to the range of 1-16 kPa water tension. On the other hand, many plant species and variations (for example, Cannabis sativa) cannot tolerate continuous saturation. Because of capillary rise and the perched water table effect, the HCM at the bottom of the container retains water much longer than the HCM higher in the container and can saturate the bottom roots for several irrigation cycles. Accordingly, the grower must determine when to water and how much to water.

For the example, I will set the minimum water volume slightly above the initial wilting point – one-fifth the difference between container capacity and initial wilting point.

minimum water volume = initial wilting point volume + (0.2 × (container capacity volume – initial wilting point volume) )

minimum water weight = initial wilting point weight + (0.2 × (container capacity weight – initial wilting point weight) )

Setting a range of minimum volume and weight values will make scheduling easier and allow flexibility for weather variability. In the above example, the top of the range would be:

minimum water volume = initial wilting point volume + (0.5 × (container capacity volume – initial wilting point volume) )

minimum water weight = initial wilting point weight + (0.5 × (container capacity weight – initial wilting point weight) )

Determining the correct irrigation volume is highly important because under-watering will shorten the window of optimal water availability and over-watering will present resource losses and costs due to nutrient losses. Because these HCMs are densely loaded with nutrients, leachate from bottom drainage will constitute a serious loss and will necessitate compensating fertigation, raising overall costs.

The irrigation volume and weight will equal the difference between container capacity and minimum water volume and weight.

irrigation water volume = container capacity volume minimum water volume

irrigation water weight = container capacity weight – minimum water weight

3.1.4. Establishing weight and volume at container capacity and minimal water volume during various stages of plant growth

As the plant grows, container capacity and available water decrease, requiring more frequent watering at less volume to maintain constant levels of available water. Growing roots realign HCM particles and occupy pores, displacing air in the largest pores and readily available water in the largest water-filled pores, thereby decreasing available water and air-filled space. In addition, root weight adds to container weight, without contributing to available water, further distorting the relationship between weight and available water.

The grower will have to adjust irrigation volume and timing accordingly. Making these adjustments requires determining the decreased water volume and weight at container capacity and at the minimal water volume and weight.

3.2. Gravimetric system for commercial operations

For large-scale commercial operations, where precision matters, using a gravimetric system initially requires correlating container weight to container capacity (CC), readily available water (RAW), initial wilting point (IWP)and permanent wilting point (PWP) through periodic stages of plant growth.

A large commercial operation might require statistical precision in correlating weight to available water, requiring an adequate sample and analysis of variance. I have outlined the general procedure for this determination without reference to sample design. Please contact me for this guide.

3.3. Spatial irrigation patterns and water distribution

The optimal target for irrigation is even moisture distribution throughout the rhizosphere. However, vertical distribution will always segregate with height due to capillary rise, and horizontal distribution will vary somewhat in a fabric bag due to uneven evaporation and watering variation. A volume of water applied rapidly infiltrates more horizontal space than the same volume applied slowly, which penetrates more deeply. Thus, an optimal application will delay the downward movement of water and spread the application area with a rapid application of the water volume. An optimal irrigation pattern will distribute water with slightly more volume at the outside edges and with broad horizontal coverage, insofar as that is practical.

* I wish to acknowledge my debt to The Rev, author of the book, True Living Organics. The zonal system I describe directly descends from his work on layers and spikes.