I don't want to plant 12 lettuce plants and 8 spinach plants all at once. I would never be able to use that amount of food at the time of harvest. But, if I plant a couple greens this week and then a couple more next week and then a couple more the next week, I get a constant harvest and will be able to have fresh greens regularly from my garden throughout the summer.
I got a little ambitious with the first planting, but with an early harvest and a late harvest, these greens will be able to be harvest and eaten over 3 weeks.
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The next lettuce/spinach/collards will be planted in front of the existing plants and will be done soon.