Soil Temperature and Precipitation Affect the Rooting Ability of Dormant Hardwood Cuttings of Populus

In addition to genetic control, responses to environmental stimuli affect the success of rooting. Our objectives were to: 1) assess the variation in rooting ability among 21 Populus clones grown under varying soil temperatures and amounts of precipitation and 2) identify combinations of soil temperature and precipitation that promote rooting. The clones belonged to five genomic groups ([P. trichocarpa Torr. & Gray x P. deltoides Bartr. ex Marsh] x P. deltoides ‘BC’; P. deltoides ‘D’; P. deltoides x P. maximowiczii A. Henry ‘DM’; P. deltoides x P. nigra L. ‘DN’; P. nigra x P. maximowiczii ‘NM’). Cuttings, 20 cm long, were planted in Iowa and Minnesota, USA, in randomized complete blocks at 1.2- x 2.4-m spacing across three planting dates during 2001 and 2002. Soil temperatures were converted to belowground growing degree days (GDD) (base temperature = 10°C) accumulated over 14 days. Genomic groups responded similarly for root dry weight, number of roots, total root length, and mean root length, that increased as belowground GDD increased. Belowground GDD and precipitation governed rooting throughout the 14-d growing period. A minimum of four days above 14°C, along with sufficiently dispersed precipitation (e.g. no more than 3 d without a precipitation event), were needed to sustain aboveaverage rooting. Therefore, we recommend using a base temperature of 14°C for future models estimating belowground GDD in northern temperate zones.