I am gardening for the first time, and so far so good! We have planted a few spinach plants, and have harvest three times from them in the last month. How long will the spinach continue to grow and produce in a climate of 80-90 degrees during the day, full sun, and the plants being well watered?
Spinach is a crop that likes the weather to be on the cooler side. Spring sown spinach will generally begin to go to seed when the days become long and the temperatures are consistently in the upper 80s to 90s. It may also "bolt" (go to seed) if the plant is not getting enough moisture. You will be able to tell "bolting" is occurring when the central stem becomes taller and rougher and the spinach leaves become much smaller. Once bolting occurs, the plant is pretty much done for the year. However, you can plant spinach again in the late summer for a fall and possibly winter crop if your winters are not too harsh.
There are also other plants that provide the same sort of food which do not mind longer summer days and warmer temps - and even a bit of drought. One of these is "vegetable amaranth" - a culinary relative of pigweed. Another is lambs quarters, a common edible weed. Then there are more exotic replacements, such as Malabar Spinach, a tender vine that just gets even more revved up and productive when the temperatures begin to rise.