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Harvest the romaine heads about 65–70 days after planting the seeds. Cut the entire head off above the base if you want a second harvest. Give the lettuce time to re-sprout leaves after the first harvest. Pull the romaine head out of the ground to ensure a single harvest. Break any clumps of dirt off of the roots. Break apart the lettuce head and rinse the individual leaves.
When grown from seeds, romaine lettuce takes only a little less than 3 months to fully mature. You can tell when the heads are mature by their visual appearance: they’ll have a dark green color and look leafy and open. Unlike iceberg lettuce, the romaine heads will not be densely closed in upon themselves when mature. If you’d like to harvest the entire romaine head at once, use a sharp pair of gardening shears to snip through the base of the romaine. Make the cut about 1 inch (2.5 cm) above the surface of the soil. Be careful not to cut through any rocks or soil, or you’ll end up dulling the blades of your shears. When you cut off the entire head at once, the roots of the romaine will often produce additional lettuce leaves. After they grow and mature, you’ll be able to glean a second harvest. You can expect to wait another 55–60 days for the second harvest. However, these leaves will not form another “head” shape, and will be looser and less numerous than the lettuce leaves on the initial romaine head. If you’d rather not have a second harvest of lettuce, you can harvest the entire head of lettuce at once. You won’t need gardening shears for this. Just grasp the base of the lettuce head with one hand, and firmly pull upward until it dislodges from the ground. Pulling up the full romaine head will bring the roots out of the ground as well. In order to leave the garden patch relatively undisturbed, and to avoid bringing dirt indoors, remove excess dirt from the romaine’s roots. You can do this with either hand while you’re pulling the lettuce head up from the ground.  Once the lettuce is uprooted, pat the dirt back in place so there isn’t a hole left in the garden patch. You can also dig around in the soil a bit to remove any leftover roots that may be stuck in the soil. If left in the soil, these leaves may re-sprout and grow more romaine. Once you’ve brought the lettuce head inside, break it apart by pulling each individual leaf away from the head’s base. Then rinse the individual leaves under cool tap water. You can serve the lettuce immediately in a garden salad, or keep the leaves for up to 10 days in an airtight bag in your fridge.