Onion bolting has occurred when the plant has formed a seedstem. Onions bolt in response to the size of the plant and in response to cool temperatures. The critical size for bolting is reached when the plant reaches the five-leaf stage. Cool temperature effects are cumulative. Sufficient cool temperatures (often experienced in the last three weeks of June) will then induce bolting in some varieties.
The time of seeding will have an effect on possibility of bolting. If onions are seeded early and a warm spring results in rapid early growth, many onions will reach the critical size for bolting. Those onions seeded last may not grow large enough to be induced to bolt during cool temperatures (i.e., in June). Transplants and sets are especially susceptible to bolting, as they reach sufficient size early in the season. Different varieties of onion will exhibit different levels of bolting.
SOURCE: Hortopics: 8/85. B. Zandstra Michigan State University Dept. Of Horticulture