Plant Competition Experiment

Plants are an integral part of the environment and with out them our lives would be greatly diminished. Have you ever gone without food for more than one day? Can you imagine a world without denim jeans or cotton shirts? Thanks to plant scientists and crop producers, these are worries that most of us never encounter. Much of the bounty we enjoy in the United States directly results from the great successes of American agriculture. The need to continue that success and provide a high standard of living for all people has created large demand for a new generation of highly skilled plant scientists.

Plant scientists are a diverse group of professionals that study plants on the molecular, cellular, whole-plant, and plant-community levels. One sub-set of plant scientists are the weed scientists, which study the effects of weeds on other plants and effective ways to manage weeds in crops, lawns, parks, lakes, and other ecosystems.

One research area of particular interest to weed scientists is plant competition. Plants are both affected by and capable of altering their environment. In a community of plants, competition takes place when one or more environmental resources such as light, moisture, CO₂, and mineral nutrients becomes limiting. The competitiveness of a plant is its relative ability to obtain a specific resource when in competition with another plant. The level of competition is dependent on the crop, weed, and environmental factors, as well as interactions among these factors. Many of our most troublesome weeds are problematic becuase they scavenge resources at the expense of the crop in which they are growing. One example would be weeds in a lawn. Weeds that grow low to the ground persist there because they benefit from frequent mowing and are highly competitive with lawn grasses for light, water, and nutrients.

This experiment illustrates competition between weeds and a crop. It is intended to answer several questions regarding interactions among weeds and crop yield. One question that can be asked is - what is the effect of weeds when they emerge with the crop and are allowed for defined periods of time? A second question is - what is the effect when the crop is kept weed-free for certain periods of time and then weeds are allowed to grow? By answering both of these questions we can determine the period that a crop must remain weed-free if we want to keep weed competition from reducing yield. This period is defined as the "critical weed-free period." Weed scientists perform these types of experiments for a variety of crops to determine the best timing for weed control.

The materials for this experiment are readily available at most variety stores. The plants that are used include radish as the crop and birdseed (millet and other crop seeds) to simulate weeds. Radish was select for several reasons: 1) it produces a harvestable yield in a relatively short period of time, 2) the radish bulb provides an easily harvested and measured product, and 3) it is not a very competitive crop, so the effects of weeds are easily seen. A millet-based birdseed mix was chosen because it is readily available in the pet food aisle of most stores.

(Click here to see photo of supplies)

Area with full sun or artificial light for growing plants (patio, greenhouse, or bench with grow lights)
Four (4) gallons (0.5 cu. ft., 15 L) of potting soil mix - greenhouse mix of sphagnum peat, vermiculite, perlite, and a wetting agent
Eight (8) plastic flat trays (10 inches x 20 inches recommended)
Pot stakes and permanent marker
Radish seeds (about 150 seeds)*
Millet seed (available as birdseed in most discount stores, wild birdseed that also contains sunflowers, sorghum, or other grains will work as well)*
Furrowing tool
Tablespoon
Fertilizer (Click here for recommendations on fertilizer)
Paper towels
Balance or food scale

*Note: Due to various flat sizes available the following plant populations are recommend:
Radish Seeds Planting Population	Radish Seeds Thinned Population	Weed Seed (Millet)
11 seeds/one square foot	8 seeds/one square foot	7.0 grams or 1.5 tablespoons/one square foot

Treatments

Control (no weeds)
2 week competition: weed seed added at planting and weeds removed two weeks after planting
3 week competition: weeds removed 3 weeks after planting
4 week competition: weeds removed 4 weeks after planting
6 week competition: weeds not removed until harvest
1 week delayed competition: add weed seed 1 week after radish seeding
2 week delayed competition: add weed seed 2 weeks after radish seeding
4 week delayed competition: add weed seed 4 weeks after radish seeding

Week	Instructions
1	Fill 8 flat trays with potting soil. Level off each tray and tamp lightly. Prepare label stakes with treatment information, date, and any other information that you deem important. Assign a treatment to each flat. Make two to three lengthwise rows in the soil with the furrowing tool; plant radishes evenly in each row. Sprinkle millet seed evenly over the soil surface in the flats for treatments 2, 3, 4, and 5. The control tray and delayed competition trays do not receive millet seed. Cover the crop and weed seeds with a layer of soil 1/4-inch deep. Tamp down lightly. Follow the same procedure when planting delayed competition experiments in following weeks. Water gently as needed to keep the soil moist, but not overwatered. Add time release fertilizer to each treatment according to the manufacturers recommendations one week after planting.
2	Thin radishes in all flats to 5 seedlings per row by clipping at the base. Add weed seed to treatment 6. (One week old radishes without weeds) (One week on radishes with weeds.)
3	Remove all weeds from treatment 2 and maintain this tray weed-free for the remainder of the experiment. Add weed seed to treatment 7.
4	Remove all weeds from treatment 3 and maintain this tray weed-free for the remainder of the experiment.
5	Remove all weeds from treatment 4 and maintain this tray weed-free for the remainder of the experiment. Add weed seed to treatment 8.
6	Harvest all trays. Gently remove the radish plants from the soil and strip the remaining soil from the roots. Wash the soil from the radish roots and pat the radish dry with paper towels. Weigh all radishes from each flat with a balance or food scale and record.

The best way to present the data from this experiment is with a graph (click to see an example graph). By graphing the data, you, as well as the readers of your figure, may gain important insights that were not redily apparent form the data alone. The graph could use the fresh weight of the radishes as the Y-axis data and weeks after planting for the X-axis. You could use two lines, one where weeds germinated with radishes and one where weeds germinated after the radishes. Treatments 1 and 5 could be used as controls for both lines. From this graph you should be able to interpret the time period in which the radishes should remain weed free in order to obtain a high yield. In your graph, this period will be indicated by weeks where the weeds diminished the fresh weight.