We use Mach-Zehnder interferometry to measure ice-crystal tip radii and growth rates in dilute aqueous solutions of trehalose over a range of concentrations. Although trehalose at high concentrations is a well-known inhibitor of ice crystal growth in aqueous solution, we find that the ice instead grows faster with the trehalose concentrations below about 8 wt%, with a peak tip velocity near 2 wt%. By directly measuring the 3D morphology of the growing ice crystals with the interferometer, we find that the dendrite tip in solution becomes thinner and sharper than that in pure water, causing the more rapid growth of ice. (C) 2018 Elsevier B.V. All rights reserved.