[header] How to harvest romaine lettuce [title] Harvest the romaine heads about 65-70 days after planting the seeds. [step] 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. 
[substeps] Unlike iceberg lettuce, the romaine heads will not be densely closed in upon themselves when mature. [title] Cut the entire head off above the base if you want a second harvest.