Iris rhizomes were dug and graded by size. One hundred rhizomes, avg. wt. 23.4 g., were singly potted in 6 inch plastic standard pots. The pots were plunged, to the pot rim, into fresh sawdust, in an open coldframe.
After 4 weeks in the coldframe, and at subsequent two week intervals, pots were brought into a greenhouse for forcing. Half of each group was forced under long day conditions by night break, following a natural daylength. The other half received continuous lighting. Forcing studies were terminated when irises in the field bloomed.
Bloom dates were similar under long days and continuous light treatments. The highest percentage of plants bloomed under long days. Stalk heights at anthesis averaged 12 to 18 in. Stalk height decreased as natural cooling time increased in plants under long days, but not under continuous light. Plants with shorter flower stalks also had shorter foliage.