ReGrow Green Free Grant Project Report
1Department of Physics, University of Texas at Austin,
204 East Dean Keeton St. Stop C2200, Austin, TX 78712, USA
The Regrow Project was an undergraduate student research project funded by the University of Texas at Austin Green Fee Grant. In this experiment, we grew food from scrap hydroponically while measuring the crop yield, water and energy consumption. Two trials were conducted, the first in the Fall of 2017 where lettuce was regrown from lettuce stumps, and the second in the Spring of 2018 where basil was regrown from cuttings. In both trials, the same plant was grown from seed in identical settings to compare water and energy consumption between growing plants from seed versus growing plants from food scrap. In both trials, it was found that plants did regrow from food scrap, however they did not hold any advantage over growing plants from seed. Plants grown from seed used around the same amount of water an energy as those grown from food scrap. Plants grown from seed also showed a higher yield percentage than plants grown from food scrap.
It is known that some common foods will regrow into new plants from their food scrap1. These foods, specifically broccoli, cauliflower, onions, potatoes, lettuce and other root vegetables and leafy greens, can potentially re-grow into new plants assuming there is enough meristematic tissue2 remaining on the food scrap. In early trials of this experiment, we had the most success sprouting new roots in water. With this, we decided to perform the experiments in hydroponic systems3.
Trial 1: Regrowing Lettuce
For the first trial of the experiment, we decided to re-grow lettuce from lettuce stumps. Lettuce is considered a winter crop and we expected it would do well in the late Fall. The first trial of this experiment was conducted in the old greenhouse atop the Welch Building on UT Campus. Because the Welch Building was under demolition at the time, a backup power supply and internet source were required to conduct the experiment.
System Design, Trial 1
The design of our hydroponic system featured two grow tubs (one for regrown food scrap and the other for control plants grown from seed), that drained into a filter bucket (to capture large sediment, such as coconut coir grow media), and then finally to drain into a water reservoir. From the reservoir water was pumped back into the grow tubs to re-circulate the water. Stand- ing pipes were placed in the grow tubs so water levels could fluctuate between 25 to 30 gallons, mimicking a “flood and drain system” (known to produce optimum plant growth as it exposes roots to more oxygen without drying them out)4. See Figure 1 below:
To encourage rooting, we covered the base of the lettuce stump in hormonal powder. The stumps were then placed in orchid baskets filled with expanded clay pebbles5 for support. A plastic mesh was wrapped around the baskets to keep the clay pebbles in place. The baskets were then secured into a styrofoam board which floated in the reservoir tub. The temperature, pH and conductivity levels of the water we measured continuously using BlueLab pH, Conductivity and Temperature probes placed in the water. Liquid fertilizer6 was added to the reservoir tubs as nutrients to the plants. Ideal conductivity (or nutrient) levels are around 1 EC 7, and when this fell below the optimal, value more liquid fertilizer was added. Ideal pH levels are around 7 M 7, and when this level fell or rose too far from 7, pH Up (35% Phosphoric Acid) or
pH Down (35% Potassium Hydroxide) solutions were added to the water reservoir. See Figure 2 below:
Figure 2: Trial 1 Setup
Unfortunately in the recirculating tub system, we saw significant leaks at the fixture points. With this we decided to use a single tub system only and to instead use an air bubbler and small submersible pump to keep water flowing inside the tub and to add oxygen. See Figure 3. The regrow plants from were left to grow in this setting for 28 days from October 20th to November 17th 2018. Afterwards the control plants were left to grow in the same setting for 28 days from November 1st to November 29th 2018. The reservoir water level, pH, conductivity (nutrient level), temperature and growth height were monitored every 2-3 days during this period of time.
Figure 3: Trial 1 Single Tub System Design
Results and Outcomes, Trial 1
Plant growth for Trial 1 was measured by the height of the plant. For our first trial with the regrown lettuce, we saw five out of eight lettuce stumps regrow into new plants. We ran this trial for 28 days and observed a yield of 5 plants until the last 2 days where we saw a yield of three plants. For the plants grown from seed, we saw six out of eight seeds grow into full lettuce heads. However, due to unexpected disruptions in the experiment from system leaks, we could not make a clear comparison of water and power consumption from the regrow plants to the control plants. See Figure 4, 5 and 6 below:
Figure 4: Trial 1 Results
Figure 5: Final height of Regrow Plants after 28 days.
Figure 6: Final height of control plants after 28 days.
For regrown lettuce plants, very little water was added to the system. The energy usage for the system was estimated to be the sum of running all of the power consuming components of the system for 28 days, specifically the submersible pump, the bubbler, the Bluelab guardian for the sensors, the Raspberry Pi computer, the AT&T hotspot for internet, and the backup generator for reliable power supply. See Figures 7 and 8 below:
Figure 7: Water consumption for Regrow Plants, Trial 1
Figure 8: Power consumption for Regrow Plants, Trial 1
Our greatest challenge for Trial 1 was fighting leaks at the junctions between the grow tubs and PVC pipe in our original two tub design. Despite the use of bulkhead fittings and rubberized paint at points of contact, leaks continued to occur. We believe this is due to the plastic of the tubs being too thin. These Sterilite tubs (made of plastic #5, Polypropylene) are designed to be storage bins and not to hold liquid. For the second trial of our experiment we decided to rebuild the recirculating system with containers designed to hold large amounts of liquid.
The great city of Austin, Texas is famous for having very unpredictable weather. Despite planting our lettuce in October, we experienced spikes in temperature up to 90 degrees Fahrenheit for several weeks during the trial. Lettuce and other winter crops thrive in temperature ranges of 60-65 degrees Fahrenheit 8. When temperatures exceed this range, lettuce and other winter crops will produce inedible flowers instead of lettuce heads8. With this, we did see significant growth in several of our lettuce plants, however this growth went towards producing lettuce flowers instead of lettuce heads.
Towards the end of the first trial, we saw algae9 begin to appear in the tubs with the plants. Algae can be harmful to plants and aquatic life in that it competes with other organisms for oxygen and nutrients in the water. Before the algae appeared in our grow tubs, five of the eight plants were regrowing. However, with the algae appearing we noticed our yield reduced from five to three plants. We do not claim this correlation is causation for our loss in yield. There are many other factors that could have contributed to this. However, for our second trail we aimed to reduce the amount of water exposed to sunlight in hopes to prevent algae growth.
Trial 2: Regrowing Basil
During the Winter Break of 2017/2018, the Welch Greenhouse was torn down and all research was moved to the new greenhouse on top of the Norman Hackerman Building on campus. This new greenhouse provided atmospheric heating and cooling, and reliable electricity for research projects.
System Setup, Trial 2
For Trial 2 of the Regrow Project, we decided to grow basil plants as we were nearing summer and the weather was warming up and basil does well in higher temperatures 10. ReGrow basil plants were grown from basil cuttings while control basil plants were grown from seed. To encourage rooting, basil cuttings were dipped into a rooting hormone powder, as was done in Trial 1. The cuttings and seeds were placed inside a peat moss plug11 which was placed into a grow bed filled with expanded clay pebbles5 for support. To measure plant growth, we decided to measure plant weight at the beginning and the end of the trial using a small scale. We felt this was more representative of biomass than in Trial 1 as some plants did not grow very tall, but did grow wide. All basil cuttings and basil seeds weighed less than a gram at the beginning of the trial, so we considered our starting weight to be zero grams.
To avoid leaks, we decided to purchase two hydroponics grow kits from Brite Ideas Hydroponics Store in Austin using the remaining funds from the Green Fee account. One kit was purchased for regrowing basil plants from cuttings and the another was purchased for basil plants grown from seed. The kits each feature a grow bed, two standing pipes, and locking hoses connected to the standing pipes for pumping water in and draining water out. Water is pumped in from a reservoir tub underneath the grow bed containing a submersible pump. Liquid fertilizer6 was added to the reservoir tubs to feed the plants as in Trial 1. Bluelab pH, conductivity and temperature sensors were also placed in the reservoir tubs to measure pH, temperature and conductivity levels. The pump was plugged into a timer so every 45 minutes the pump would turn off and water from the grow bed would drain from the hose back into the reservoir. This created a flood and drain system4 in the grow bed which is optimal for exposing plant roots to oxygen without over drying. The water level of the system was marked in the reservoir tubs using a water-resistant marker. The water level for systems were measured by the water level in the reservoir tubs when pumps were turned off. Both the regrow and the plants from seed were left to grow in this setting for 36 days from April 27th to June 2nd 2018. The water level, pH, conductivity (nutrient level) and temperature of the reservoir tubs were monitored every 2-3 days during this period of time. See Figures 9 and 10 below for Trial 2 system and setup:
Figure 9: Trial 2 Setup
Figure 10: Trial 2 Setup continued
Results and Outcomes, Trial 2
The water consumption of the regrow plants and control plants were almost identical. We believe the primary reason for water loss in the reservoir tubs was due to evaporation from the lids being open. The power consumption for the regrow and control beds were also identical. See figures 11,12 and 13 below for water and power consumption:
Significant energy was saved in Trial 2 due to the removal of the backup power generator. The new greenhouse provided reliable electricity so for Trial 2 the backup generator was no longer needed. Our energy analysis does not include the energy required to cool the greenhouse as this data was not available. It is most likely that cooling energy for the greenhouse over the summer would have contributed to the majority of energy consumption.
The basil plants grown from cuttings and the basil plants grown from seed gained almost the same amount of biomass within the 36 day trial period. It was observed that the plants grown from seed had stronger root systems than those grown from cuttings. The average biomass of the plants grown from seed was 32.4 grams, whereas the average biomass of plants grown from cuttings was 25.6 grams, despite having a ”head start” biomass at the beginning of the trial. We believe that despite having small biomass, the basil seeds contained significantly more meristematic tissue than the basil cuttings, allowing them to gain comparative biomass after 36 days. See Figures 14 and 15 below:
Both Trial 1 and Trial 2 of the ReGrow Project showed that plants can in fact be regrown from food scrap if they contain enough meristematic tissue to do so. However, both trials also showed that plants grown from seed grew just as fast, with similar water and energy consumption, while showing higher yields of healthy thriving plants. Growing plants from seed proved to be more efficient and effective, as mother nature has shown us for the past 470 million years12. However, it was still very beneficial to see that some plants can regrow from food scrap, making for a potentially more effective use of food waste.
This project was an excellent learning experience for real world challenges in engineering systems. Unexpected leaks, power outages, and unpredictable weather patterns gave way to practice engineering solutions to continue on with the experiment. We were surprised to find how little water the plants themselves required for growth and that most water loss was due to leaks and evaporation. We also greatly underestimated the importance of maintaining ideal temperatures for plants to grow, otherwise plants will grow in undesirable ways, such as producing flowers instead of foliage. When using natural lighting, a lot of heat from the sun’s rays can get trapped inside the greenhouse. As such, we anticipate a great deal of energy went into the greenhouse’s cooling system to keep the rooms at a reasonable temperature for plants to grow. Unfortunately this data was not available to us to analyze this energy usage.
Between trials, all grow tubs, clay pebbles, and probes had to be cleaned thoroughly to kill all algae, fungi and bacteria that had appeared during the trials. The grow tubs and clay pebbles had to be soaked in bleach and algaecide to ensure they were sterile enough to be used in another trial. After this, the grow tubs and clay pebbles had to be thoroughly rinsed to remove all residual bleach and algaecide that could kill the plants in the following trial. As such, a lot of water was used to simply clean and sterilize the grow beds and grow media to prepare for the following trial. En estimated 93 gallons of water per system per harvest was used just for cleaning alone. It was brought to our attention that this amount of water wasted could be nontrivial in a commercial hydroponic facility that utilizes hundreds of grow beds. Furthermore, the wastewater that was dumped from the reservoirs contained fertilizer, bleach and algaecide, all of which could cause serious damage to a receiving stream if not treated properly.
The ReGrow project was an excellent learning experience for first hand knowledge in engi- neering and science. We are eternally grateful to the Green Fee Grant for making this learning experience possible.
A very big thank you to undergraduate research assistants Maneill Parekh and Natalie Ozor for their assistance in conducting these experiments. I would also like to thank green house manager Shane Merrell for his expertise and assistance with this experiment. Thank you to Dr. Upshaw and Dr. Webber for advice and guidance on this project. And of course a very big thanks to Green Fee Grant for providing the funds to make this project possible!
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