Family Name: Amaranthaceae (formerly Chenopodiaceae)
Spinach contains a high level of calcium, but it also contains oxalic acid which binds with calcium in the diet and prevents the tortoise from absorbing and using it. In addition, it possesses a high level of calcium oxalate crystals which contribute to the formation of kidney stones. Some of the calcium oxalate is in the form of needle-shaped crystals called raphides, and when consumed in large amounts these can irritate the skin and mucous membranes in the mouth and throat.
So although Spinach is not toxic as such, and small amounts are unlikely to cause a tortoise any great damage, given the potential it has to limit calcium intake and cause internal irritation, we do not recommend that people feed it to their tortoises.
Do not confuse with Tree Spinach, which is a different plant altogether.