Rhubarb leaves are ready to harvest when they reach a certain size, which varies depending on the variety. As you can see from the chart below, there is no hard-and-fast rule for this process. Even though some varieties have been bred specifically to produce tender leaves at an early stage of development, others will be mostly green and still more so when they’re harvested. This also depends on how much sunlight your rhubarb receives during its growing season. Typically, it takes three weeks after planting before the first set of new leaves appear at the tops of stalks that are just 4 or 5 inches tall—a week later than most gardeners realize!
The keys to harvesting rhubarb leaf stalks in peak condition: Make sure young plants receive enough light; harvest when individual stems form buds rather than fully formed stalks; and pick only mature but unripe greens (which should not be eaten). After these measures have been taken, all you need to do is wait until late summer/early fall and then eat! The earlier you begin picking them off your plant(s), the larger and richer their flavor will become as well as their naturally occurring antioxidants (see page 103). Store picked rhubarb in plastic bags in refrigerator crisper drawers until ready to use. Use within 3 days or frozen for up to 1 month.