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During my research on Human Plant Interfaces I figured out music and sound is a very often used expression for this kind of interaction. As far as I know John Lifton and Richard Lowenberg are one of the first artists that used plants for creating a soundscape. Their installation "Green Music", created for the much discussed movie "The Secret Life of Plants" (movie), uses the approach of bio sensing and converts the signals to synthesized sounds. The visitors can interact with the plants via touch for changing the pitch level of a sound. Futhermore, the "Hashimoto Experiment" with a cactus by Dr and Mrs Kenneth Hashimoto caused some big attention, too. Their science oriented  presentation of their experiments and the same pseudo science language of the book "The Secret Life of Plants" damaged the image of bio sensing with plants very hard. It became very quite about Human Plant Interfaces. Nowadays, it seems that media and sound artists re-experience this technology for their artworks. I will give you a summary of projects ordered by characteristics of user interaction.

 Direct interaction with a plant:

• Kraft Test Drummie and Robert Plant: touch interface on a Plectranthus australis
• Mosszillator: stroking, pressing, squeezing or caressing interface with moos
  
• Whining Plant (jump to 2:50min):  Touch Interface with a palm
• Genesis of Biosynthia II (jump to 2:28min): Plant + object interface
• Pulsu(m) Plantae: Touch, Sound, and Light interface (Galvanic isolation and?!)
• From the hills: Touch interface with a cactus (contact microphone)
• Mogees: Touch, Stroking and gestural interface (piezo-transducer | vibration)
• Botanicus Interacticus: Touch, Stroking and gestural interface (capacitive frequencies)
• Plantas Parlantes: several touch and stroking interfaces
• Akousmaflore: several touch and stroking interfaces (capacitive approach)
• Baumarktmusik: several touch interfaces with different plants
• The Plant Ochestra: several touch and stroking interfaces

Interaction with the fruits of a plant or with vegetables:

• Frishasin pijamas: touch interface with a cone
• Noisy Cauliflower: Touch and object connection interface (resistance approach)
• Teardrop: Touch Interface with fruits and vegetables (resistance approach)
• Sauerkraut Synthesizer (video): touch and voltage interface (battery approach)

Technology review

In my opinion this list of projects summarizes nicely how plants can be used as an input device. The technology behind these projects reaches from super simple to pretty complex technology. A big number of these projects used the simple resistance approach. They measure how conductive the plants are. You can really easily rebuild it with an Arduino and a 1mOhm Resistor.

For more advanced interaction like (pressure sensitive) touch and simple stroking interaction, plants can be used as capacitive sensor. A capacitive sensor is also very easy to build and connect with an Arduino. Another option for detecting simple pressure senstive touches is the usage of a contact microphone. It is a great tool for measuring the vibrations inside a plant. These vibrations can be caused by a human or other external influences (e.g. wind). Contact microphones are a good solution if you are not familiar with Physical Computing (e.g. Arduino). The microphone signal can easily be used in sound environments (mixers and synthesizers) or the audio input of your computer.

The technology behind the gesture driven interaction, which is used by Botanicus Interacticus and Mogees, is much more advanced. Both systems depending not only on hardware, they also use a pretty complex gesture recognition software. In my opinion the gesture recognition software is the key, and not really the hardware. However, I will give you a short introduction into these two different approaches. The technology behind Botanicus Interacticus is called Touché and was developed by Ivan Poupyrev and Chris Harrison. Touché uses an advanced capacitive sensing approach, which is called Swept Frequency Capacitive Sensing. They measure a predefined range of frequencies from an object and then the gesture recognition software determine if the gesture or interaction is correct or not. Luckily, some great DIY Hackers rebuild the system with Arduino. The Arduino solution is not as accurate as Touché - it does not have such wide frequency range - but it is still very useful. The tutorial on instructables and the videos (1 | 2 | 3) are a very good motivation for rebuilding it. In contrast to Touché, Mogees analyzes vibrations instead of a complex capacitive sensing. A small piezo-transducer (contact microphone) converts the vibration of the physical object into an electric signal. This electronic signal is analyzed by their own developed gesture recognition software. I am sure they are analyzing the Frequency Spectrum of the sound signal. If I am wrong, please let me know. The advantages of the vibration approach are the low costs and it is easy to map to sound driven outputs.

All yet mentioned techniques for sensing plants are not really specifically related to plants. They also work great on other common objects. For this reason, the signals are not really send from the plant itself. The signal is influenced by the form and the kind of plant. It is not the real bio sense metabolism the plant sends when it is touched. These touche signals of a plant are normally very weak and difficult to measure. Furthermore, it is more interesting to understand the approaches, which Leslie Garcia used in her installation Pulsu(m) Plantae (article). It seems she is somehow measuring the plant reactions on environmental influences, like light and noise. Luckily, she made the whole documentation open source, which makes the whole approach more transparent. Unfortunately, I am not able to understand Spanish. For this reason, I don't want to say anything wrong about it with my half understanding via Google translator. It looks very promising to me and I will investigate it further.

The End

I tried to give you an overview about muscial interfaces with plants. This is my link collection of the last 2-3 years. If you know some more interesting projects related to Music and Human Plant Interfaces, please leave a comment or send me an e-mail. I will appreciate it!