The study, characterization, observation, and quantification of plant root growth and root systems (Rhizometrics) has been and remains an important area of research in all disciplines of plant science. In the horticultural industry, a large portion of the crops grown annually are grown in pot culture. Root growth is a critical component in overall plant performance during production in containers, and therefore it is important to understand the factors that influence and/or possible enhance it. Quantifying root growth has varied over the last several decades with each method of quantification changing in its reliability of measurement and variation among the results. Methods such as root drawings, pin boards, rhizotrons, and minirhizotrons initiated the aptitude to measure roots with field crops, and have been expanded to container-grown plants. However, many of the published research methods are monotonous and time-consuming. More recently, computer programs have increased in use as technology advances and measuring characteristics of root growth becomes easier. These programs are instrumental in analyzing various root growth characteristics, from root diameter and length of individual roots to branching angle and topological depth of the root architecture. This review delves into the expanding technologies involved with expertly measuring root growth of plants in containers, and the advantages and disadvantages that remain.

Container production of plants use substrates that are formulated to have adequate physical properties to sustain optimal plant growth; however, these properties can change over time as a result of substrate settling and root growth of the growing plant in the container. An apparatus (rhizometer) was developed that measures the changes caused by plant roots on physical properties of substrates during crop production in containers. The design of the rhizometer included a clear core, which allowed for observing and measuring a range of root system characteristics in situ, including total root length visible along the rhizometer. Physical properties of planted and fallow rhizometers were measured, and the effect of four species on substrate physical properties was determined. There was a general decrease in substrate total porosity and air space (AS) over time with both fallow and planted rhizometers as a result of both settling of the substrate and root growth into the substrate. Container capacity did not change over time with or without roots. Plants with large root systems such as Begonia × hybrida acut. decreased AS over time, whereas plants of Rudbeckia hirta L. with a smaller root system did not have the same effect. Measured total root length was highly correlated to the total dry root mass of Tagetes erecta L. and Zinnia marylandica D.M. Spooner, Stimart & T. Boyle plants. This may allow tracing and measuring root lengths to be another (alternative) method to measure root systems. Planted rhizometers also allowed easy access for viewing the root system non-destructively, providing the ability to observe and measure root growth.

An apparatus was developed that allows for a range of non-destructive measurements on root growth in containers (pot culture). The mini-Horhizotron was designed to measure root growth of small plant material such as seedlings, herbaceous plugs, or woody plant liners normally grown in containers less than 3.8 L. The mini-Horhizotron design has three chambers extending away from the center that could be filled with the same substrate or filled separately with different substrates/treatments to observe root growth response from a single plant. The objectives were: 1) to test the suitability of the mini-Horhizotron’s design and its effects on plant growth with several different species; 2) to test two different experimental designs on the mini-Horhizotrons for research purposes; and 3) to test the effect of wood-amended substrates on root length of a single species. Measurement included quantification of the longest roots growing away from the center (where the plug was transplanted). Herbaceous and woody plants grown in the mini-Horhizotrons included: Echinacea purpurea (L.) Moench ‘Prairie Splendor’, Chrysanthemum L. ‘Garden Alcala Red’, Rudbeckia hirta L. ‘Becky Yellow’, and Ilex crenata Thunb. ‘Steeds’. These plants produced root and shoot growth similar to plants grown in traditional greenhouse containers with approximately equal heights and volumes, allowing for root observations in the mini-Horhizotrons to be considered simulations of traditional container-grown crop production. Results from the initial root growth measurements provide evidence that the mini-Horhizotron may be used with a different substrate in each chamber, effectively altering a portion of the rhizosphere of one plant and reducing the number of mini-Horhizotrons needed for replications during scientific studies. Root growth was measured in three substrates containing by volume 70:30 peat:perlite (control), peat:pine-wood chips, or peat:shredded pine wood. For the species grown in pine-wood chips or shredded pine wood-amended substrates, root growth equaled or exceeded that observed in the control substrate at all time periods. The mini-Horhizotron was used to non-destructively measure treatment/substrate effects on root growth while providing full visual access to the root zone and developing root system.