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Measuring Plant Growth

To capture enough data on the overall health of your plants, we recommend that you record at least one final weight measure, one measure of root health, and all of the observation measurements that pertain to the type of plant you are using.

Weighing Plants: Fresh vs. Dry Weight

Root Mass

Root mass is recommended as a final measurement as the plant must be removed from its growing medium in order to capture accurate data. There are quite a few different methods for measuring root mass depending on the type and structure of the roots

Root Shoot Ratio

Roots allow a plant to absorb water and nutrients from the surrounding soil, and a healthy root system is key to a healthy plant. The root:shoot ratio is one measure to help you assess the overall health of your plants. Your control group of plants will provide you with a "normal" root:shoot ratio for each of your plant types, any changes from this normal level (either up or down) would be an indication of a change in the overall health of your plant. It is important to combine the data from the root:shoot ratio with data from observations to get an accurate understanding of what is happening with your plants. For example, an increase in root:shoot ratio could be an indication of a healthier plant, provided the increase came from greater root size and NOT from a decrease in shoot weight. To measure the root:shoot ratio:

  1. Remove the plants from soil and wash off any loose soil.
  2. Blot the plants removing any free surface moisture.
  3. Dry the plants in an oven set to low heat (100° F) overnight.
  4. Let the plants cool in a dry environment (a Ziploc bag will keep moisture out) - in a humid environment the tissue will take up water. Once the plants have cooled weigh them on a scale.
  5. Separate the root from the top (cut at soil line).
  6. Separately weigh and record the root and top for each plant. (Dry weight for roots/dry weight for top of plant = root/shoot ratio)
  7. The root/shoot ratio can be calculated for each treatment.
  8. Plants contain mostly water, so make sure you have a scale that goes down to milligrams since a dry plant will not weight very much.

Observation

There are many different features of a plant that can be measured through observation to determine the extent of plant growth/health. The following table describes some of the measures that you can make and also recommends how frequently you should make these observations during the course of your experiment.

  Measurement Procedure Frequency of Measurement
When starting with seeds First Cotyledon Record the number of days from planting to the emergence of the first cotyledon ("seed leave(s)" that are the first to emerge from the ground). Once
  Percentage of seeds that germinate Calculate the percentage of seeds that germinated under each of the variables in your experiment. Once
When starting with young plants Plant height
  • Measure the height of the main plant from the border of the container to the top of the main plant stem.
  • Note: you do not want to measure from the top of the soil, as the soil may condense with watering over time.
Every 2-3 days
  Number of leaves (indicates a plant's physiological age) Counting Leaves:
  • Count and record the number of leaves on each plant.
  • Count every visible leaf on the plant, including the tips of new leaves just beginning to emerge.
  • You may want to place the plant over some graph paper to avoid counting errors.
Every 2-3 days
  Surface area of leaves
  • Method 1: Trace the leaves on graph paper and count the squares covered to give you an estimate of the surface area for each leaf. Repeat this for each leaf on a plant and for each plant in your experiment.
  • Method 2: Trace out each leaf on paper. Make sure to use the same type of paper every time AND make sure that the paper is not wet. Cut out the leaf tracings and weigh them. Weigh the cutouts and divide the total weight by the number of leaves to give you the average leaf area for each plant. Repeat this for each of the plants in your experiment.
  • Method 3: Digital image analysis: Using a digital camera capture an image of a plant. Using special software, analyze the surface area of the leaves.
Every 2-3 days
  Plant color Record any observations on changes or differences in plant color. Every 2-3 days
When you are using flowering plants these two measurements serve as an additional indication of plant health 1st Flowering Record the number of days since initial planting to the first flower. Once
  Number of Flowers Record the number of flowers on each of the plants. Buds should be included in your flower count. Every 2-3 days

Bibliography

Note: All the resources listed below were originally consulted as references when creating our Science Buddies Measuring Plant Growth guide; however some of the resources no long exist on the Internet.