Every year, teachers and students come to me looking for both hydroponic and garden project ideas. Most have a limited budget and an unclear idea on how and where they can cut corners before they end up with a mess that could turn both teacher and students off of gardening.

The right knowledge is power, as one teacher I know found out. Her entire school did a project of soda bottle lettuce units, but out of 18 classrooms, hers was the only one that succeeded. The biggest factor of her success? She had the right info at hand.

Inverted soda bottle units

Inverted soda bottle units are an incredibly satisfying project. Here are five essential tips for teachers on how to make these units a success.

  1. Replace the wick with a mixture of expanded clay growing medium and coco fiber. It doesn’t biodegrade like a wick does, and I have found much better success with the coconut mix.
  2. Do not use small bottles as the failure rate is much higher with these. A 33.8 oz. water bottle is ideal.
  3. Cut the bottle and invert the top. This is better than using a 2 in. net pot sitting on top of the bottle. This allows both the teacher and the student a lot more grace, especially when the plant is small and the weather is warm.
  4. Cut the bottle properly. This cannot be stressed enough as it impacts the whole unit. Once inverted, the bottle’s neck should almost reach the bottom. Be sure to leave the cap on and to put a hole in it.
  5. Use green soda bottles as they prevent algae growth in the bottle unit that happens frequently.

I prefer the bottle unit for teachers because it’s inexpensive to make multiple units, which allows each student to have their own pet plant. This consideration of the budget from the beginning also offers the opportunity for teachers and student to have fun with the plant variables. Teachers can switch it up or continue experimenting throughout the year at almost no cost.

The inverted water bottle system…with variations

Once a classroom has mastered the inverted water bottle system, it’s time to experiment! These school projects ideas are easy, fun, inexpensive and guaranteed to work.

Light spectrum

Gather three boxes, about 1 ft. x 1 ft. x 1 ft. Cut off the top and the side facing you. From the florist, purchase 3 ft. each of red, yellow and blue transparent cello-film. Place the colored film over the portion of the box you cut away. Place a plant in each box so the students can see how each light spectrum affects the plants.

Teachers can also build a box with clear film and place a straw between the box and the edge of the cello film. Let the kids exhale into the box. This adds extra CO2 for the plant. For comparison, have another plant outside the box receiving regular air.

pH scale highs and lows

This experiment uses three of the inverted water bottle units. Mix up a gallon of nutrients and add nutrient water to each unit. Drop the pH of the nutrient water in the first unit to 4, make the second unit have a pH of 6 and raise the last unit to the pH of 8. This demonstrates the affects of above optimal and below optimal pH on plants against a control plant.

Oxygen content in water

This project uses two bottle units. Give each student a straw. Have them take turns oxygenating one of the water bottle units by lifting the plant and blowing into the water throughout the school day. This demonstrates the difference between stagnant water and aerated.

Growing medium and moisture

Using as many bottle units and mediums as the imagination can hold, put a different growing medium into each of the units. This shows students that the plant needs oxygen in the right amounts at the root zone, in addition to demonstrating how the various mediums hold water differently.

Healthy nutrients vs. junk food

Using three bottle units, mix one bottle unit with nutrient water, one bottle unit with plain water and one with Kool-Aid, soda or tea. This demonstrates to students that plants need specific food and have a specific diet.

These same basic experiments can grow with the students. As they get older, you can go from a “show and tell” format of learning to asking them to observe and explain why and how these experiments worked. They can even get involved in build-offs.

Hopefully, I have helped inspire you to share your knowledge and creativity as the kids—the teachers of the future—see garden experiments that work!