Archaeologists and Geologist use standardized photo scales that are place in the photo to give a sense of scale, and sometimes even color cast, to the object they are photographing. This set, made for foresters, includes seven scales that range in size from 1 meter down to 3 cm. Surveyers also have expensive scales with spikes on one end to allow them to be driven into the ground, but they are precisely marked and expensive. You don't need anything near that comprehensive for trees only 1 foot high. You could attach a spike, such as a tent stake, to a yardstick made out of weather resistant materials and drive it into the soil near the trees. Add some guy wires held down by smaller spikes if needed and it should last for several years. This aluminum yardstick has two convenient holes to attach your spike and guy wires. You could even paint alternating units (such as inches, or 5cm marks) in contrasting dark and light colors.

As Matt Grum pointed out in a comment to the question, if you just want to place something in the scene that is universally recognized as being a certain size a 12 oz. Coke can is near universal pretty much anywhere in the world.