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A very low cost way to provide a durable plant stress detection device is to use popsicle sticks, a 1 inch x 2 inch section of purple gel filter, a glue stick, and a marking pen. Popsicle sticks are available at many office supply stores (approximately 300 for $1.50 to $2.00).
List of purple gel filters that may be used for detecting plant stress:
Handles of sticks may be used for decorations or to write the basic rules for viewing plants: Dark = Healthy Plant; Light = Stressed Plant