Sonderegger pine (Pinus x sondereggeri H.H. Chapm.), the natural hybrid of longleaf (Pinus palustris Mill.) and loblolly pine (Pinus taeda L.), commonly occurs in longleaf pine seedlots grown in forest tree seedling nurseries in the southeastern United States. Because longleaf pine seedlings have a grass stage with minimal epicotyl development (< 1 cm), the initiation of stem growth (12 to 15 cm) in longleaf pine seedlots has been used to indicate that hybridization with loblolly pine (Pinus taeda L.) has occurred. Sonderegger pine seedlings are typically culled at the nursery due to observations of poor form and wood quality after outplanting. However, research documenting Sonderegger pine seedling morphology has not been published for more than 60 years, and to our knowledge, no seedling quality assessments have been made. To better understand how morphological traits of longleaf and loblolly pine are expressed in hybrid seedlings, stem length, hypocotyl length, and root collar diameter (RCD) were compared among one-year-old container-grown longleaf, loblolly, and seedlings visually classified as Sonderegger pine. Sonderegger pine seedlings had a range of stem development, with most (62%) seedlings measuring < 12 cm tall. Some pure longleaf pine seedlings had up to 10 cm of stem elongation, but the cause of early height growth in these seedlings is unknown. More than 90% of Sonderegger pine seedlings met or exceeded RCD recommendations for planting loblolly (≥ 3.2 mm) and longleaf pine (≥ 4.75 mm).