Whether you’re growing deciduous or evergreen crops, field stock or container plants, timing means everything during fall fertilization applications.
Nursery production using high rates of fertilization may reduce frost hardiness in azaleas, even if fertilization is terminated well before the first frost, according to research at the University of Georgia. A team of researchers led by Frank Henning studied the effects of fall fertilization on cold hardiness and growth of Rhododendron x
Providing 75 ppm N from Aug. 1-Sept. 29 and stopping fertilization six weeks before the predicted frost date was considered the industry standard for this study, according to Henning.
Measure twice
“When compared to the industry control, cold hardiness of leaves from the November harvest was not affected by fall fertilization, Henning said. “However, providing fertilization at 125 ppm regardless of whether or not fertilization was extended through September or November, reduced frost hardiness of stems in the November harvest.”
Cold hardiness was not different from the control when fertilization was either discontinued at the end of July or applied at the 75 ppm rate through the November harvest.
A 125 ppm rate of fertilization applied in either late summer, or in both late summer and fall, reduced evergreen azalea stem frost hardiness in November. In addition, higher (125 ppm) fertility through October and November continued to suppress frost hardening through December.
In contrast, extending fertilization at 75 ppm N through October and November did not reduce fall or early winter frost hardiness.
The bottom line: Maintaining a moderate fertility rate, growers may be able to realize the benefit of fall fertilization, such as potential increased growth the next year, without any reduction in cold hardiness, Henning said.
Evergreen versus deciduous
Tennessee Technological University’s Nursery Research and Service Center in Cookeville, Tenn., studied thetiming of fall fertilization and subsequent spring growth on an evergreen (Ilex crenata‘GreenLuster’) and deciduous (Forsythia x intermedia‘LynwoodGold’) container-grown woodyplant.
“One of the most critical periods nutritionally for container-grown plants is the fall of the year before the onset of winter,” said horticulture student and study leader Denita Hadziabdic. “Often the use of eight- to nine-month controlled-release fertilizers in the early spring results in sub-optimal tissue nutrient content by fall.”
In this study, fall fertilization treatments included 14-7-7 nursery grade granular fertilizer, MgO (magnesium oxide), 0-0-50 and an untreated control.
There were significantly different growth responses between the two species, Hadziabdic said. Spring growth of forsythiawas influenced most through nitrogen fertilization.When nitrogen was applied in the spring prior to flushing, growth was increased up to 35 percent regardless of fall fertilization treatment.
Spring growth of the evergreen increased 11 percent with fall nitrogen applications. But there was no significant growth increase on Ilex with a spring nitrogen application.
“This may be due to the plants being harvested prior to full emergence of new growth,” she said.
Spring growth of forsythiawas 10 percent greater with fertilization treatments applied on either Nov. 4 or 18. Spring growth of Ilexwas similar in relation to fall application timing.
“For optimizing springgrowth,deciduous species likeForsythiaxintermedia‘Lynwood Gold’ would benefit from fertilization applied late in the fall, while evergreen species likeIlex crenata‘GreenLuster’ benefit from fall fertilization regardless of application timing. Spring applied nitrogen provides a nutrition boostto aidoptimum growth over and abovefallfertilizationstrategy,” Hadziabdic said.
The basics
Growers who use granular and liquid fertilizer programs should begin withholding fertilizer about six weeks before the average first frost date, said Ted Bilderback, nursery extension specialist at North Carolina State University. Growers who use slow-release fertilizers may have trouble reducing the plant growth late into the season.
“Tender growth caused by high or fluctuating rates of nitrogen fertilizer late in the season may also prevent hardening,” Bilderback said.
But the source of nitrogen is also important. Nitrate nitrogen is readily available to plants for uptake. However, high nitrate levels may stimulate new shoot growth.
“Most information on wintering nursery crops suggest increasing potassium levels to promote the cell permeability, which is important in avoiding cellular freeze damage,” Bilderback said. “Although information on this practice is conflicting, maintaining adequate tissue potassium levels is advised.”
Potassium levels in foliage should have an index of 50 to 75 for most woody ornamentals and a soil test index value of approximately 50 is adequate. If foliar or soil levels are well below these values, winter hardiness may be improved by applying potassium.
“If you wish to apply a complete NPK fertilizer in the fall, wait until above-ground plant parts are fully dormant. After deciduous plants have dropped their leaves, a fall fertilizer application is usually safe. A moderate level of balanced fertilizer should not cause plants to break dormancy or reduce hardiness,” Bilderback said.
Arbor Valley Nursery in Brighton, Colo., stops irrigating in mid-September so plants don’t get a new flush of growth in the field, said Dave Edmundson, Arbor Valley owner.
“So if we get a late September rain, that can sometimes hurt us,” Edmundson said.
The nursery typically stops applying slow-release fertilizer Aug. 10.
“Late fertilization pushes out new growth and it doesn’t harden off quickly enough,” Edmundson said. “It generally takes six weeks for new growth to harden off.”
The region’s average killing frost is Oct. 15.
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For more: Frank Henning, University of Georgia, (706) 613-3640; fhenning@uga.edu.
- Kelli Rodda