For The Answer
Deadheading And Renewal Pruning Affect Subsequent Bloom Of Chaste Tree
Garry V. McDonald, Michael A. Arnold, and Jerry M. Parsons
Dept. of Horticultural Sciences, Texas A&M University, Mail Stop 2133, College Station, TX 77843-2133
Index Words: deadheading, chaste tree, landscape maintenance, renewal pruning, Vitex agnus-castus.
Nature of Work: Large flowering clones of Vitex agnus-castus L., such as ‘LeCompte’, ‘Shoal Creek’, and ‘Montrose Purple’, are being promoted as a summer alternative for lilac (Syringa vulgaris L.) for southern U.S. climates (2). The overall effect of the flowering of these cultivars is somewhat reminiscent of lilacs in bloom, but they have the potential to withstand the summer heat and low winter chilling of the southern regions. As an added benefit, repeat bloom during the same growing season sometimes occurs on this species (1, 2).
Despite these positive landscape characteristics, V. agnus-castus suffers from some limitations including a tendency to grow too large for many small residential home sites, a tendency to accumulate a number of dead twigs and small branches in the interior canopy over time, and a potential to produce copious amounts of seed that could potentially contribute to weeds in the surrounding cultivated or non-cultivated landscapes (1). Removal of inflorescences after flowering would reduce the potential for seed development and dispursal. The objectives of this research project was to determine the effect of various pruning severities after flowering flushes on the subsequent ability of plants to produce additional flowers in the same season and during the initial bloom in the year after pruning.
Rooted cuttings of Vitex agnus-castus ‘LeCompte’ were growth in #1 (2.3-L) black nursery pots (Nursery Supplies, Inc., Fairless Hills, PA) and planted in May 2003 to raised landscape beds in College Station, TX. Beds were weeded and irrigation was applied as needed. In Feb. 2004, all plants were trimmed back to within 15 cm of the ground and allowed to grow until flowering occured in early June 2004. The number of inflorescences were counted on each plant and the length of the primary axis on each inflorescence was measured. Four pruning treatments were then imposed: 1) no pruning (control), 2) deadheading by removal of only the spent inflorescences, 3) pruning each flowering stem to one half of its original length, or 4) pruning the entire plant to within 15 cm of the ground. This process was repeated following bloom in August 2004 and September 2004. Inflorescence number and length were again determined in May of 2005 to access the residual effects on the initial bloom of the subsequent year following pruning.
Results and Discussion: A significant (P ≤ 0.001) interaction among time and pruning treatments was found for both the number of inflorescences and length of the primary inflorescence. Prior to imposition of the pruning treatments, all the groups of plants assigned to the various treatments did not differ in inflorescence number or length (Table 1). During the year plants were deadheaded following flowering, deadheading either had no effect (inflorescences number and length in August and inflorescence length in September) or increased (inflorescence number in September) flowering compared to non-pruned controls (Table 1). Deadheading also increased flower length in the spring of the subsequent year (May), but decreased the number of flowers in the subsequent year (May). Pruning stems to half their length had only minimal impact on inflorescence length, but substantially reduced the number of inflorescences produced in subsequent bloom cycles, completely eliminating a third bloom in September of 2004 (Table 1). Pruning plants to within 15 cm of the ground following bloom, caused severe reductions in the number and length of blooms in subsequent cycles, and like the half height pruning treatment eliminated a third late summer flush of flowers in September 2004 (Table 1).
Significance to Industry: Deadheading Vitex agnus-castus ‘LeCompte’ eliminated the potential for seed production without substantial reduction in flower number or loss of inflorescence size. Severe pruning of this cultivar should curtailed if reductions in flowering are to be avoided. This practice documents an environmentally friendly way to cultivate this durable landscape ornamental shrub.
1. Arnold, M.A. 2002. Landscape plants for Texas and environs, sec. ed. Stipes Publ. L.L.C., Champaign, IL. p. 1094.
2. INSERT DR. PARSON’s TNLA GREEN ARTICLE on VITEX here.
Table 1. Interactions among time after pruning and extent of pruning of Vitex agnus-castus ‘LeCompte’ planted in landscape beds in College Station, Texas, n = 16.
Length of primary
Number of infloresences
Date Treatment inflorescences (cm)
June 2004 Non-pruned control 36.8 ax 22.8 a
Deadheady 42.0 a 22.7 a
Prune to half height 45.1 a 21.4 a
Prune to within 15 cm of ground 36.7 a 21.8 a
August 2004 Non-pruned control 23.2 a 17.9 a
Deadhead 26.9 a 17.1 a
Prune to half height 9.1 b 15.3 a
Prune to within 15 cm of ground 1.1 b 8.7 b
Sept. 2004 Non-pruned control 11.6 b 21.0 a
Deadhead 28.2 a 18.2 a
Prune to half height 0.0 c 0.0 c
Prune to within 15 cm of ground 0.0 c 0.0 c
May 2005 Non-pruned control 78.3 a 18.0 b
Deadhead 57.3 b 21.6 a
Prune to half height 12.5 c 22.4 a
Prune to within 15 cm of ground 3.6 c 8.8 c
Month ***z ***
Pruning treatment *** ***
Month x pruning treatment *** ***
xMeans within a date and column followed by the same letter are not significantly different using a least squares means test at P ≤ 0.05.
yDeadheading indicates removal of spent inflorescences only immediately after flowering, whereas, other pruning treatments included cutting limbs back to half their original length after flowering or pruning all stems to within 15 cm of the soil immediately after flowering.
z*** = significance at P ≤ 0.001.