Air temperature during cutting propagation of cold-intermediate and -sensitive crops can be reduced if root-zone heating is provided

Asexual propagation of herbaceous ornamentals occurs from late winter to early spring coincides with seasonally low daily light integrals (DLIs) and average daily temperatures (ADT) outdoors. Supplemental lighting and air and root-zone heating (RZH) can be used to promote rapid callus induction and rooting of cuttings. Due to the range of greenhouse crop base temperatures (cold-tolerant, cold-intermediate, and cold-sensitive), one strategy is to reduce the air ADT of certain crops while using RZH as a potential energy saving strategy. Therefore, the objective of this study was to quantify how reduced air ADT and increased RZH temperatures influence root initiation and development of cold-intermediate and-sensitive herbaceous annual and perennial crops propagated by shoot-tip cuttings. Unrooted vegetative cuttings of angelonia ‘Archangel Raspberry’ (Angelonia angustifolia), lantana ‘Landmark Sunrise Rose Improved’ (Lantana camara), New Guinea impatiens ‘Clockwork White Improved’ (Impatiens hawkeri), scaevola ‘Scalora Brilliant’ (Scaevola aemula), thunbergia ‘Sunny Lemon Star’ (Thunbergia alata), verbena ‘Cadet Upright Red’ (Verbena × hybrida), geranium ‘Moxie! Deep Rose’ (Pelargonium interspecific), Kwik Kombos ‘Pink Lemonade Mix’ [petunia ‘Dekko Star Rose’ (Petunia × hybrida), lobelia ‘Techno Heat Light Blue Improved’ (Lobelia erinus), and calibrachoa ‘Callie Yellow Improved’ (Calibrachoa × hybrida)] and portulaca ‘Cupcake Carrot’ and ‘Colorblast Lemon Twist’ (Portulaca grandiflora) were inserted into a soilless medium and placed in a greenhouse with an air ADT of 21°C and RZH set point of 24°C to callus. After 6 d, trays of cuttings were placed between two greenhouses with an air ADT of 16 or 21°C and among 3 benches which provided RZH set points of 21, 24, or 27°C. After 14 or 16 d post-callus, measurements were collected for all genera and geranium and portulaca ‘Colorblast Lemon Twist’ were transplanted into a common environment. The air ADT and RZH individually influenced stem length of angelonia, lantana, kwik kombos, portulaca, thunbergia, and verbena, and interacted to influence angelonia and lantana. Under an air ADT of 21°C, angelonia was 30%-45% taller than an air ADT of 16°C. Stem length of geranium, New Guinea impatiens, and scaevola were not influenced by air ADT or RZH. With an RZH set point of 21°C, the root dry mass of thunbergia was 72% greater under an air ADT of 21 than 16°C. Lantana shoot dry mass was 23% smaller under an air ADT of 16°C and RZH of 21°C compared to an RZH of 24°C. Finished plants of geranium and portulaca were not influenced by air ADT or RZH and were comparable in size and date of marketability between treatments. Due to little to no differences in crop timing or quality, the majority of the crops studied can be propagated with an air ADT and RZH of 16 and 24°C, respectively, without negative effects during or after propagation.