The phenology of onion bulb development at different sites and its relevance to incomplete bulbing (‘thick-necking’)

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Abstract
In three successive seasons onion bulbing and maturity from simultaneous sowings occurred two to three weeks earlier on a sandy loam at Wellesbourne, Warwickshire (latitude 52.2°N) than on a silty soil at Holbeach, Lines (latitude 52.8°N). This difference was associated with 1–2°C higher soil temperatures and faster early seedling growth at Wellesbourne. In previous seasons high percentages of ‘thick-necked’ onions had occurred at the Holbeach site. Analysis of data from cultivar trials conducted at several sites over many seasons showed that the occurrence of a high percentage of ‘thick-necked’ plants was associated with sites, seasons or cultivars of late maturity date. Late sowing delayed bulb development and maturity, and increased nitrogen levels slightly delayed bulb development. However darkening the soil surface, thereby slightly increasing soil temperatures, had no effect on growth and development.