Prehypertension and mild hypertension affect an estimated 157 million U.S. adults. Cardiovascular disease (CVD) risk and associated mortality is elevated in this population. Treatment options are limited consisting of lifestyle modification, which is often ineffective, or drug therapy, which carries risk of side effects. Highly safe, efficacious, and acceptable treatment options for this population are needed. Hawthorn standardized extract (HSE) is approved for use in Europe to treat heart failure, and preliminary evidence suggests it may have a blood pressure lowering effect. However, prior trials of hawthorn have based dosage recommendations on animal studies. Therefore, the investigators propose a dose-finding study to measure the pharmacodynamic effect of three doses of standardized hawthorn extract and placebo.