Thursday, July 14, 2011

Notes on softwood propagation

Blueberries are propagated by vegetative cuttings (not by seeds). Most blueberry cultivars are easily rooted under mist from either hardwood cuttings (collected in the dormant season and stuck in April), or from leafy softwood cuttings taken in summer.  The time for taking softwood cuttings will be here soon (around August 1).  Here are a few tips for increasing your rooting success and avoiding disease problems.
Softwood cutting ready for sticking in the rooting bed

Design a well-drained rooting bed – Failure of cuttings to root is often due to poor drainage rather than to disease.  Rooting beds, especially open-air beds exposed to rainfall, must have nearly unlimited drainage capacity.  This requires a coarse, open rooting medium (ground pine bark is preferred) over a well-drained base.  Build beds atop an 18-inch layer of coarse sand, with drain lines (sock pipe or tile) to ensure rapid movement of water through the bed.
A small outdoor rooting bed with a sand base for improved drainage.  Note windbreak!

Water quality – Before going to the expense of building a propagation bed, have your water tested by the North Carolina Department of Agriculture make sure it is of adequate quality for rooting cuttings.  Rooting is generally good with water from shallow wells in the coastal plain, even though some wells in southeastern NC have excessive levels of bicarbonates or iron that accumulate as a residue on leaves.  Some of the best wells for propagation have been deeper wells in Bladen and surrounding counties;  however, Sodium can be a problem in coastal counties, where saltwater intrusion has been observed in wells with depths of 160 to 200 ft.  On sites where saltwater intrusion is a problem, it may be necessary to use only shallow well water (from wells 50 to 60 ft deep) for propagation.

A large commercial rooting bed with pin-type mist nozzles
Bed design and rooting substrate – Two-by-eight-inch boards are used to construct beds of various lengths, 4 feet wide and 8 inches deep, filled with milled pine bark.  Wider beds are not recommended because they are difficult to reach across for sticking cuttings or for weeding.    Do not line the beds with plastic or weed barrier fabric, as this will inhibit drainage – if a bottom lining is needed, use 1/4-inch mesh screen wire (galvanized hardware cloth).   The pine bark rooting substrate should be in direct contact with the underlying sand.  One common mistake is re-using old bark.  Do not re-use old rooting media!  It is infested with fungal pathogens that will carry over and cause disease in the next batch of cuttings.

Scouting for disease-free cuttings -- Before taking cuttings, walk the field looking for disease symptoms, and avoid taking cuttings from infected bushes.  Blueberry red ringspot virus, Blueberry stunt, and Necrotic ring blotch virus are transmitted via propagation.  These diseases are visible in August when softwood cuttings are taken, so be sure to look for and avoid them.  Blueberry stem canker is a fungal disease that can be transmitted via infected hardwood, and to a lesser extent, by softwood cuttings.  Canker-infested fields can be identified visually and should also be avoided.  

Trueness-to-type – unintended cultivar mixing within a field can cause serious problems, especially when mixed-up cultivars have different ripening seasons.  While scouting for diseases, watch for and exclude any mixed or mislabeled rows.

Patented cultivars -- Propagation of patented cultivars without a license is illegal.  Patented cultivars include Star, Rebel, Emerald and many newer cultivars.  Non-patented cultivars include O’Neal, Legacy, Duke, Reveille and most older cultivars.

Collecting and handling – Softwood cuttings are usually collected around August 1 in southeastern NC, but optimal timing may vary by a week or two depending on growth stage, cultivar and location.  Cuttings should be five to seven inches long, leafy semi-hardened new growth, not fully mature but firm enough to be pushed into the rooting media without breaking.

Softwood cuttings are best collected in early morning and can be cut with pruning shears or broken by hand. If present, tender, succulent shoot tips can be pinched off to avoid wilting and decay.  Lower leaves are stripped off by hand, keeping the upper three to four leaves.  While collecting cuttings, do not allow them to wilt – sprinkle with ice water and bag cuttings in coolers in the field as soon as they are gathered.  Stick softwood cuttings about two-thirds of their length into the rooting media, and begin misting immediately.  Cuttings are usually spaced 1.5 to 2 inches apart in the rooting bed.
Softwood cuttings under mist

Mist timing and monitoring – The method used for watering cuttings during rooting is known as “intermittent mist propagation”.  Cuttings are misted for four to five seconds at a time, with the cycle repeating every five to ten minutes, using solenoid valves, timers and horticultural mist nozzles.  Daily misting in the summer begins around 9 AM and ends an hour or two before sunset.  Misting is not needed at night.  After the cuttings have been stuck in the beds, water management becomes critical.  The goal is to keep the cuttings from drying out, while at the same time not saturating the rooting medium.  If you can pick up a handful of rooting medium and squeeze more than a drop or two of water from it, it is too wet!

Water management for hardwood cuttings -- Misting of hardwood cuttings begins with leaf budbreak in April, and continues until cuttings are rooted in July.  After rooting, water is gradually reduced for the remainder of the summer to prevent drowning and avoid disease problems.  Hardwood cuttings are generally more forgiving of different watering regimes, and misting/watering protocols vary widely between growers.

Pin-type mist nozzles are easily maintained
Water management for softwood cuttings -- Misting of softwood cuttings begins immediately when the cuttings are stuck and continues for six to eight weeks until the cuttings are rooted, at which point watering can be gradually reduced. Leafy cuttings are very sensitive to drying -- during the first six to eight weeks, failure of the mist system for more than an hour or two on a hot day will result in complete loss of the cuttings.

Extending the propagation season -- some growers erect hoop supports and fully enclose late-summer softwood rooting beds with white plastic covers.  This high-humidity enclosure provides some shading, allows rooting without continuous use of intermittent mist, and keeps the beds warmer so that root development continues later in the fall.  Another modification is to use a greenhouse or high tunnel to exclude rainfall – this allows the use of less well-drained rooting media, and again extends the time of year during which cuttings will root.  Caution:  Covered or enclosed propagation beds can become too hot and “cook” the cuttings – proper timing and constant monitoring with careful use of shade and ventilation will be needed.

For further reading:
Krewer, G. K. and W. O.Cline. 2003. Blueberry Propagation Suggestions.
Bilderback, T. E., R. E. Bir and M. A. Powell 1993. A Simple Intermittent Mist System for Propagation

(This article previously published in the NC Blueberry News)