Defoliating Woody Cut Stems

Cut flower growers using woody branches to enhance their marketing mix may have difficulty getting clean, leaf-free branches.  Currently, growers are either removing the leaves by hand or waiting to cut until after the first frost has naturally defoliated the plants.  A few growers have experimented with applying chemical defoliants, such as those used to defoliate cotton, with limited success.  We experimented with several defoliants, applied in the field, and with different temperatures applied postharvest, to ascertain their effects in promoting leaf drop.  In addition, we looked at trying to promote defoliation after harvest through temperature manipulation.
    
In the first experiment, conducted in September 2002 with curly willow (Salix matsudana ‘Tortuosa’) growing in Raleigh, North Carolina, we applied Burnout (containing vinegar and lemon juice) at 1500 or 2000 ppm, Captain™ (chelated copper) at 100 or 200 ppm, crop oil concentrate (COC) at 1% v/v, Florel® (ethephon) at 250 or 500 ppm, Harvade® (containing dimethipin, a cotton defoliant) at 1600 ppm plus 1% v/v COC, Scythe® (pelargonic acid, a naturally occurring substance found in numerous foods and animals) at 100 or 250 ppm, and a tap water control.  The defoliants were selected with an eye towards naturally-occurring chemicals that would have low human toxicity, allowing them to be easy to use.  We included Harvade, which is not as environmentally friendly and should be used with caution, because we had heard that it was effective in trials a grower had conducted.
    
All plants were sprayed to runoff with a backpack sprayer.  Air temperatures ranged from 66F to 75F, relative humidity ranged from 75 to 90%, and no precipitation occurred for 12 days after defoliant application.  In this experiment, Harvade applied at 1600 ppm produced significant leaf drop, inducing 85-90% necrosis and desiccation of foliage 6 days after treatment (Fig. 1).  Slight agitation of the stems resulted in leaf drop.  Ethephon applied at 500 ppm produced 10% foliage chlorosis, but the foliage did not exhibit further chlorosis, necrosis, or drop.  No other treatments produced foliage yellowing or leaf drop.  We concluded that chemical concentrations were too low to produce defoliation.
    
For the second experiment, twelve treatments were applied to bittersweet (Celastrus scandens) and winterberry (Ilex verticillata ‘Winter Red’) in Soldiers Grove, Wisconsin in October 2002, before fruits had fully colored.  Plants were sprayed to runoff.  Air temperatures ranged from 42F to 59F; relative humidity was 90 to 100%, and a rain event occurred 2 hours after defoliant application.  Higher concentrations of the defoliants were used, including: Burnout at 2000, 3000, and 4000 ppm; Captain at 200, 400, and 800 ppm; Florel at 1000 ppm; Harvade at 1600 ppm plus 1% (v/v) COC; Scythe at 250, 500, and 1000 ppm; and a tap water control. Ten days after spraying, the plants were evaluated.  In this experiment, only three treatments produced significant defoliation of bittersweet: Captain at 800 ppm (100% defoliation), or 400 ppm (50% defoliation), and Florel at 1000 ppm (36% defoliation). Florel encouraged fruit ripening and opening, but no treatment significantly increased fruit drop. These results were good, but may the waxy layer of the leaf more pliable and speed movement of chemicals through the cuticle.

For the third experiment, American beautyberry (Callicarpa americana) plants were sprayed to runoff using a backpack sprayer in the field in Raleigh, North Carolina in October 2003.  Treatments that showed promised from the earlier experiments were used at higher concentrations, including Captain at 800, 1000, or 1200 ppm; Harvade at 1200, 1400, 1600, or 1800 ppm; Florel at 1000, 1500, 2000, or 2500 ppm; and a tap water control.  Air temperatures ranged from 63F to 70F, relative humidity ranged from 59 to 76%, and no precipitation occurred for one day after defoliant application.

In this experiment, all treatments except for Florel at 1000 ppm provided greater defoliation of beautyberry than the control (Fig. 2).  All concentrations of Harvade provided equal defoliation, as did the 800 and 1000 ppm concentrations of Captain (Figs. 3 and 4).  Florel is usually more effective when temperatures are higher than 60F, while Captain and Harvade are somewhat less sensitive and can be effective at temperatures as low as 55F.   Also, treatments were applied on an overcast day.  Previous research has shown that cloudy weather reduces response to some defoliants.  High temperatures and direct sunlight at the time of application make to runoff using backpack sprayers.  Plants were observed daily for two weeks, and again 6 weeks after treatment to assess permanent damage to the plants.  Harvade was the only defoliant that promoted defoliation (88%), and no irrigation treatment promoted defoliation.  None of the defoliants caused plant death.

Postharvest defoliation.  In Oct. and Nov. 2002, cut stems of beautyberry and curly willow were placed for 1, 3, 5, or 7 days at 41F, 69F, or 95F and held in distilled water under cool-white fluorescent lights for 12-hour days.  After treatment, stems were placed in distilled water at 69F.  Ten days after treatment, jars containing stems were tapped three times on an extruded metal bench and percent defoliation recorded.  Holding curly willow stems at 69F significantly increased defoliation to 68%, compared to 53% for stems held at 41F and 28% for stems held at 95F.  Further, the longer the stems were held at 95F, the less foliage they lost, because the stems began to dry out.  High humidity during the storage process may be helpful.  However, no treatment promoted 90% defoliation. Holding cut beautyberry stems at any temperature did not promote leaf or fruit drop.

Conclusions – Putting This Information to Work

To summarize, of the defoliants tested, Captain at 800 ppm provided 100% defoliation of bittersweet and 76% defoliation of beautyberry with no fruit drop, while higher concentrations (1000 and 1200 ppm) did not provide greater defoliation.  A 2500 ppm concentration of Florel produced 67% defoliation of beautyberry; lower concentrations may not be as effective.  Harvade at 1200 and 1600 ppm produced significant defoliation of beautyberry and curly willow, respectively.  Plants treated with Florel and Harvade began to leaf out again after about two weeks, so cutting should be expedient.  Allowing curly willow plants to wilt before applying herbicide did not increase defoliation.  Burnout, Scythe, and crop oil concentrate did not promote significant leaf drop on any species tested.
    
We determined that defoliation was inconsistent and dependent on several factors, including plant water stress, air temperature, precipitation following application, type of chemical, concentration of application, and plant species.  Spraying well-irrigated plants on a sunny day when air temperatures are between 75F and 90F and relative humidity is higher than 85% is ideal.  In order for the herbicide to dry slowly but thoroughly, precipitation should not occur for at least 4 hours following application of defoliants.
    
Economics may also dictate what defoliant is used.  A gallon of Florel spray solution at 2500 ppm costs $5.12, whereas an 800 ppm concentra-tion of Captain copper costs $0.27/gallon, and Harvade at 1200 ppm costs $0.23/gallon.        

While our work with defoliation after harvest did not produce the results we needed, Debbie Hamrick, editor of FloraCulture International, reports that growers in the Netherlands have been successfully defoliating holly after harvest. In the Boskoop area of the Netherlands, where most Ilex verticillata is produced, growers use a combination of heat and humidity to defoliate stems.  One of the larger growers places stems on auction carts, seals the carts with plastic and places them in an old sea container where he raises the temperature to 86F.  For early harvests, the heat treatment generally takes seven to eight days followed by one week without heat.  As the harvest season progresses, stems require only three to four days at 86F followed by one week without heat.  When stems come out of the heat treatment there may be hand work to remove the few leaves that did not fall off.  During the last four weeks of harvest (late November to early December), stems do not require defoliation.  Instead of using a dedicated heat chamber, some growers stack stems on ground plastic on the greenhouse floor.  The stacks are wetted down and then covered with white plastic and finally topped with black plastic.  This less precise defoliation technique requires at least two weeks, and perhaps longer.

The authors would like to thank the Association of Specialty Cut Flower Growers Research Fund for supporting this project, John Zehrer and Alex and Betsy Hitt for supplying plant material, Uniroyal Chemical for supplying defoliant, Fafard for potting media, Dillen Plastics for pots, and Harold F. Wilkins, Scott Aker, Diane Mays, and Jessica Hyatt for assistance in defoliant application.