In our orchards, you will notice several very small plants that have just been transplanted this spring. Please avoid harvesting any leaves from these plants. Newly set plants need all their foliage to build a strong root system.
Other healthier sized plants are ready for harvesting from June until August, but to keep the plants healthy, vigorous and producing well, remove only about one-third of the leaves from a plant at any one time.
Remember, the leaf blades contain large amounts of oxalic acid and should not be eaten.
For more information: http://en.wikipedia.org/wiki/Rhubarb