Spinach: a good leaf with a real limit
Spinach is safe for rabbits, and nutritionally it has plenty to offer — iron, folate, vitamin K and vitamin A in a leaf most rabbits eat with enthusiasm. What keeps spinach out of the everyday bracket is its chemistry. Spinach is high in oxalic acid, which binds calcium and other minerals as it passes through the body, and it is also one of the more calcium-rich leaves a rabbit is likely to meet. Rabbits absorb calcium very efficiently and clear the surplus through their urinary system, so a diet that leans heavily on high-calcium, high-oxalate greens day after day is asking that system to work harder than it should. None of this makes spinach dangerous in sensible amounts. It makes spinach a rotation green: a modest handful once or twice a week, in among lower-calcium leaves, rather than the base of the daily salad.
Serving size and rhythm
For an average adult rabbit, a small handful of fresh spinach leaves — baby spinach or the larger cooking leaves both count — once or twice a week is the right scale, and on spinach days it should replace part of the usual greens rather than pile on top of a full portion of everything else. Pairing helps: serving spinach alongside low-calcium leaves such as romaine keeps the day's overall mineral load balanced. Wash spinach thoroughly, because its crinkled leaves trap grit and anything sprayed on the crop, and pick out any yellowing or slimy leaves before serving. Frozen spinach is no shortcut around the washing and portioning: it thaws into a limp, concentrated mass that most rabbits refuse, and fresh leaves are far easier to count out into a modest handful. If your rabbit has never had spinach, introduce a few leaves at a time over several days and watch the droppings, as with any new green.
Rabbits for whom spinach needs a vet's word
A rabbit with a history of bladder sludge, urinary stones or kidney trouble is the case where spinach stops being a casual decision. For those rabbits a vet may have set a deliberate low-calcium diet, and high-oxalate leaves like spinach may belong on the avoid list altogether, or in strictly occasional amounts the vet is comfortable with. The same caution applies to any rabbit whose urine regularly looks thick and chalky — that is a conversation to have at the next check-up, mentioning which greens the rabbit eats most, spinach included. For a healthy rabbit with varied greens, no such worry applies; the rotation habit is protection enough.
Keeping an occasional green occasional
Spinach's risk is drift: a leaf this convenient, sold in bags that need using up, easily slides from twice a week to most days without anyone deciding it should. Putting spinach into the written feeding plan stops the drift — in KinsLe's Care Teams the spinach days are simply the days they are assigned, each serving is ticked off as done, and the rest of the week's greens fill themselves in around them, keeping spinach in the useful, occasional place it belongs.