Choosing the Right Pot Size

Container size is an important factor for plant growers since it influences the optimization of production space. It is important for roots to have space to spread out and not become bound together (“root bound”) when growing in solid soil mediums and in hydroponic systems. Root restriction is of most concern when considering container size since reduced root volume can have large influences on plant quality and performance. Restricted root volume has physiological effects on the plant in this way. Rooting space can increase root mass independent of nutrient availability in solid growth medium and in hydroponic grow systems.

Just How Important Are Your Pots?

Container size when transplanting plants can affect the continued success of the plant when re-planted in larger containers or outdoors. Plants transplanted outdoors or to subsequent containers from larger starting containers tend to be more successful and have increased leaf area, and increased overall growth.

Consequences of Root Restriction

• Root and shoot growth, distribution, and proportion constrained and altered
• Leaf, seed, and flower production reduced
• Water utilization simulates drought stress
• Inefficient nutrient uptake and utilization
• Oxygen uptake and diffusion decreased
• Total crop yield decreased
• Photosynthetic rate declined
• Hormone production inhibited
• Delayed fruiting and maturation
• When transplanted from a small container to outdoor fields, confined roots leave soil space available for competing plant species to invade
• Root restricted container-grown plants may also be predisposed to drought stress when transplanted outdoors

Planting in Containers vs Outdoors vs Planting Beds

Container planting differs from field conditions in soil temperature, soil drying rate, soil moisture distribution, available volume of soil for root growth, and nutrient availability. Soil tends to dry out faster in containers, especially smaller ones. Most plants are highly adaptable and will alter their root volume and appearance in ways that will aid survival when resources are limited.

How Roots Change When Pots Are Too Small

Root length density is a measure that represents a plant’s ability to take up nutrients and water relative to its morphology. The density of roots can change with weather and soil aeration, especially with changes that affect nutrient dispersal and water table levels.

Root diameter is influential in nutrient and water uptake ability. When soil is not well aerated and when soil is deeper in the ground it becomes more compressed and difficult to penetrate. When soil layers are compacted, dense, and depth increases, root diameter increases to enable more penetration into the soil.

Soil compaction and water table levels are the two key factors that have an effect on root morphology and total mass of root systems.

In most plants, larger density and mass of roots occur in the upper layers of soil where oxygen and nutrients are most available. Fine roots that maintain wider and shallower contact with the soil are mainly utilized for water and nutrient acquisition. Root length density is highest in the first 10cm of soil and generally decreases with total root depth which varies based on the water table. Approximately 50% of the root system occurs in the first 50cm of soil in outdoor crops of the plant.

The plant has a taproot system where deep roots anchor the plant in the soil and take up water from deeper soil sources to combat force from wind and drought on the soil surface. The deep root profile of the plant can grow up to 200cm deep in uncompacted field soil in order to reach the water table. The straight and tapering taproots of the plant can account for more than 1 ton of root biomass per hectare, by weight they account for the majority of the total root mass.

Ideal Conditions for Roots in Container-Grown Pots

• Good moisture retention and high-quality growth medium are essential to plant success
• Drainage is very important and drainage trays should be pumped or wet/dry vacuumed to remove excess water
• Aeration is important to allow for oxygen diffusion (“breathing”), the soil should not be prone to compaction
• Proper nutrient proportions and abundance which can be aided by compost amended growth medium
• pH balance must be adjusted for when using bottled nutrients

Picking the Right Pot Size

Root growth takes place primarily during the vegetative growth phase. When roots develop abundantly during vegetative growth floral yield is higher since the uptake of water and nutrients is not limited. Small containers limit the availability of water and nutrients as well as physical space for root growth. Even young plants of several species can produce roots up to 1 meter in length which can be easily constrained by container dimensions. Doubling container size can increase the production of total plant mass (biomass) by up to 43%.

Since container-grown plants have no need to reach the water table, it does not need to expend the extra energy to grow deeper and more competitive roots. The most common recommendations to grow mature plants in 3-5 gallon containers. Different varieties may respond better to more or less space.

1.5-3 gallon containers are reported to be suitable for plants, but 5-gallon or larger containers are recommended for providing ample rooting space. Only one plant should be grown per container to prevent plants from competing for nutrients. This also helps to ensure that if conditions in one container are detrimental, fewer plants will be lost as a result.

Types of Containers for Greenhouse Plants

Plant containers are usually measured by volume which means container depth will vary depending on the width of the container. In general, a one-gallon pot is 6.5” wide and 6.25” deep. A 5-gallon bucket is usually ~12” wide and 14 ¼” deep.

Standard Pots with holes and trays for drainage are suitable; pump or wet/dry vac out excess water.

Smart Pots (fabric pots) provide more oxygen, allow roots to be pruned when they grow out the side of the fabric. They need watering more often but allow for drying/aeration from all angles At least a 5-gallon size should be used (double normal pot size to allow for more soil and more moisture retention), and smart pots need an extra-large tray to catch water drainage.

Air Pots provide more oxygen to roots, prevent root binding/allows air pruning. Air pots dry out like smart pots so more watering is needed but a regular drip saucer size may be used.

Pot Size Recommendations

A plant’s pot to biomass ratio should be less than 1 gram total dry plant weight to 1 liter of space (1 gallon = 3.8 liters) ¹³. For most plants, it is recommended to have up to 2 gallons per 12" of height. Morphology differences among varieties slightly differ but general final plant size to container size can be estimated⁴.