The effects of low-dose radiation causing DNA damage continue to be subjects of interest. Problems with existing approaches to low-dose DNA damage are that single-strand breaks (the predominant radiation-induced lesion) are very rapidly repaired and that results using current methods for measuring DNA damage can be difficult to interpret. As a novel approach, we conducted studies using plants (rye grass and the model plant Arabidopsis) exposed to X-rays and used the alkaline comet assay to measure DNA damage and repair after exposures. Consistent with previous studies, we detected so-called ‘rapid’ and ‘slow’ phases of DNA repair after acute exposures of 5 and 15 Gy. After exposures corresponding to 2 Gy and lower, ‘rapid’ repair was so fast that it was difficult to detect. We also found that the so-called ‘slow’ phase in both plants actually consisted of two components; an initial period of negligible repair lasting 80–120min followed by a period of rapid repair lasting <30min. Using Arabidopsis mutants homozygous for both ATM and BRCA1, we found that both of these genes are required for DNA repair during the 3-h period of our experiments, indicating that the ‘slow’ phase involves a homologous repair (HR) of double-strand breaks and clustered single-strand breaks. The lag of repair in the ‘slow’ phase presumably involves induction of expression of genes involved in HR repair such as BRCA1 and RAD51.