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There is no limit to your

There is no limit to your harvest, Haifa. The plants might wilt in high heat and low moisture and that could signal the end. But pick leaves of any size (always leaving a few little ones on there), keep the plants well watered, and—depending on the variety—you should have many weeks of harvest. Depending on your first frost date (if indeed you live in an area that has frost), you might even squeeze in a second season. Check your frost date here (this is set for Kansas City; key in your location on the page): http://www.almanac.com/content/frost-chart-united-states/KS/Dodge%20City

Then, if you are likely to get a fall frost, count the days that back into summer based on the growth-to-maturity period of your spinach, and plant that second crop.

Enjoy!