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In general, harvest the outer

In general, harvest the outer leaves to allow the inner ones to develop further. However, if the plant develops a large stalk with buds/flowers/seeds (called bolting), then you should pull the entire plant and use the leaves as you can. The leaves on the stalk will look narrower. When a spinach plant starts to bolt, it makes the leaves bitter. You can try to slow the bolting by pinching off the flower/seedheads as they appear, or keeping the plant moist and providing a little shade if the weather gets too warm.

 

Bolting can be caused by stress, warm temperatures, or daylight longer than about 14 hours (which happens in many areas of the US around May). Because of this, spinach is best planted in early spring or fall, when it is cooler and the days are shorter.