Bioavailability of non-heme iron is impacted by the ratio of inhibitors and enhancers present. Inhibitors include phytates and polyphenolics. Enhancers include vitamin C, citric acid and other organic acids. We now know that individuals can adapt and absorb non-heme iron more effectively over time.
Polyphenols, which include tannic acid, can inhibit iron absorption, and are found in coffee, cocoa, black tea, green tea and many herbal teas.
You should avoid these foods at meals if you are trying to increase iron absorption, eat them at a separate time from the major sources of iron in your diet.