There are many challenges and traps for the casual botanist like me to fall into when trying to identify plants, some species look so much like another that unless you have a super memory for detail making a record in the field can be quite difficult. This is the case with telling small-leaved (or in some books equal-leaved) knotgrass from its close cousin, common knotgrass. They have much in common at a casual glance and are both very common species.
There are, however, some little clues that make all the difference. First and foremost, small-leaved flowers a month later (July to November) that common knotgrass (June to November) so, in mid-summer, there is a time when there is only one to chose from. Next, common knotgrass is usually a bigger and more upright plant than small-leaved which tends to be a rather prostrate, even sprawling plant. Next, the flowers of common knotgrass tend towards being pink whereas small-leaved are usually more white or perhaps even greenish.
Small differences perhaps but they make for a totally different species!