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Plant Growth and Overcrowding
by Science Made Simple

Botany and Plant Growth Projects, Overcrowding

11. Does crowding affect plant growth?

Determine the effects of growing plants close together vs. growing plants farther apart.

Materials: 6 medium-sized pots, 10 bean seeds, potting soil, water, ruler, large measuring cup, desk lamp, pencil.


  1. Fill all pots with an equal amount of potting soil. Be sure that the soil has been dampened with water.
  2. Using a pencil, make 5 holes about 2 centimeters (cm) deep in the soil of one pot.
  3. Place seeds in each hole making sure that they are spaced relatively close but equal distance from each other within the pot.
  4. Place the remaining 5 seeds, 1 in each of the remaining 5 pots, about 2 cm deep.
  5. Cover the seeds with soil.
  6. Place all the pots underneath a large desk lamp so that each pot receives full light. Be sure to water each plant as needed.
  7. The seeds will germinate in about 7 days, and you will be able to begin making stem measurements. Take measurements for 14 days. Note the difference in stem length for each plant and write down your observations.

Results: What differences did you observe between seedlings that were crowded and those that were not? (color of leaves, length of stems, etc.) What caused those differences?

12. Roots Restrictions

Does the amount of room a plant has for roots make a difference in how big a plant will grow, regardless of how much fertilizer the plant is given?

Plant seeds in a variety of different-sized containers using vermiculite or other soil-less material, so you will be able to give each plant a measured amount of fertilizer. Or plant a number of plants in the same size containers and vary the amount of fertilizer and see what happens. Be sure to use small enough containers so that root growth really will be constricted.

Effect of Mulch

13. Does Colored Mulch Affect Soil Temperature?

To determine if different colored plastic (or mulch) on the soil surface affects the temperature of soil.

Materials: Digital thermometer; potting soil; 8 pots; colored mulch in black, white, and red; window sill with full sun; tape. For the colored mulch, you can use different paint colors to paint black plastic, or you might use different colored plastic bags or mulch from the store.


  1. Fill pots with equal amounts of soil. Place 2 uncovered pots on the window sill where there is full sun.
  2. Cover 2 pots with black plastic, 2 pots with white plastic, and 2 pots with red plastic. Make sure the plastic covers the top of the pot, and tape it to the pot.
  3. Place these pots on the window sill.
  4. In 24 hours, record your first temperature measurements. You can do this by sticking the temperature probe in the drainage hole of the pot. For 10 days, record the temperatures for each pot in the morning and afternoon. Note the differences in temperature among the colors, and record observations.

Results: Which pot had the highest soil temperature? What color kept the soil the coolest? Why do you think that some mulch colors make the soil hotter or cooler? How does this affect plant growth (see project 14)?

14. Does Colored Mulch Affect the Growth Rate of a Plant?

To determine if colored plastic (mulch) will affect the stem length of plants.

Materials: Materials: 8 pots; 8 bean seeds; colored plastic in red, black, and white; ruler; water; toothpicks; greenhouse or window that receives full sun; tape; potting soil; pencil; small cup.


  1. Fill pots with equal amounts of potting soil. Make a hole about 2 cm long in each pot using a pencil, and place 1 seed in each pot.
  2. Cover seed with loose soil. Mark where you planted the seed with a toothpick.
  3. Cut plastic in pieces large enough to cover each pot. Cut a smaller hole in the center of the plastic and cover pot with the plastic. Be sure the smaller hole in the center is where your toothpick is so the seed is exposed to light.
  4. Take the toothpicks out after you have taped the plastic to the pot. Do this twice for each of the colors. You will have 2 red, 2 black, 2 white, and 2 uncovered.
  5. After the seeds have emerged through the soil (about 5 days), take stem measurements. Measure the stems in each pot for 14 days.
  6. Do not forget to water your plants during the experiment. About every other day, use a small cup and pour a small amount of water in each hole where the plant is growing. Note the differences in height among the plants grown over different colored plastic.

Results: Which plastic helped produce the longest stem length? Which plant had the shortest stem length? What do you think this tells us about how a plant uses reflected light?