Heading behaviour of eight strains of timothy was observed in the sowing and first harvest years, after sowing on a series of dates between March and August. All strains produced heads in the sowing year. Only a very few plants failed to head when sown between 18 March and 29 April. With subsequent sowing dates the proportion of heading plants progressively declined, most markedly in S.50 Phleum nodosum, least in the early‐heading P. pratense strains. No heading occurred in the sowing year after the mid‐August sowing. The environment to which the later sowings are exposed differs from that of the earlier sown lots in that (a) the initial temperatures and daylengths are higher and (b) the decline in daylength occurs earlier in their development. One or both of these factors must be responsible for failure of heading among plants of the later sowings. The sequence of heading dates among the strains in the sowing year differed considerably from that in the same plants after overwintering, suggesting the action of some factor other than photo‐period in determining heading date in the sowing year. Variability in heading date within the strains was much greater in the sowing year than after overwintering, and increased with lateness of sowing.

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