JARS v37n2 - Growing on One-year Rooted Rhododendrons

Growing on One-year Rooted Rhododendrons
Francis Raughley

Reprinted from Valley Forge Chapter Newsletter

Azalea and rhododendron plant tissues whose growth has been stimulated by artificial light and heat during the first year of propagation need to be conditioned by a hardening-off process to help them adjust their physiological and nutritional functioning to better withstand the more rigorous environment of the cold frame or lath house in growing on - they are not ready for landscaping until after the third year.

Actions and cultural practices during this period that have proven helpful:

• Prepare a cold frame, lath house or liner bed as shading will be needed for the young plants. About half shade is advisable. Shading may be reduced or eliminated for azaleas, but not rhododendrons, after four weeks. Shading should be provided again in November for protection from the winter sun. Natural windbreaks or structural protection can be helpful in the hardening-off process but tight glass or plastic enclosures should be avoided.

• Proper preparation of the growing media for potting or planting is fundamental, yet simple. Equal parts of sphagnum peat moss, coarse sand (concrete, not bar or plaster sands), and garden loam are required for a depth of 8 to 10 inches. Good drainage is necessary and can be assured with a raised bed. Inoculation of the soil with a helpful fungus, mycorrhizae, is very important. It can be derived from woodsy soil, preferably taken from an area where ericaceous plants grow. Take a spadeful of such soil and mix it in the root zone area of the new planting. Not too distant research development may bring this helpful fungus to your garden commercially as nitrogen-fixing inoculants is today.

• Pinch or snip growing tips when planting in the spring. Some clones require repeated pinching in the summer to obtain shapely plants. Any soft tip growth should be pinched back with the beginning of freezing weather, also.

• The application of plant food well in advance of oncoming winter (August 1 to 15). A formula of 0-10-10 or 0-12-12 (no nitrogen) takes care of the phosphorus and potassium requirements. Application of this formula in granular form at the rate of 2½ pounds to which is added ¾ to 1 pound of magnesium sulfate (Epsom salts) per 100 square feet of growing area rounds out a good mixture.

• Delay mulching young plants heavily until the first Fall-freeze when some frost forms in the topsoil - usually between Thanksgiving and Christmas. This assures that bark splitting due to a sudden plunge in temperature will be minimized. The insulating effect of a heavy summer mulch if applied and not pulled back in time, would not permit the soil beneath to cool down sufficiently during the cool nights of autumn. The plant tissue at soil level and above to the depth of the mulch remains soft and tender and subject to bark splitting with a sudden plunge in temperature. However, from this time on for the life of the plants, mulching is so very important in maintaining desirable soil temperature by leveling fluctuations, controlling weeds and conserving moisture.

Excellent mulches are pine needles, leaves of oak or beech, etc. (chopping of leaves is optional), wood chips, shredded bark and sawdust. All parts of black walnut contain toxic tannin and are to be avoided. Apply mulch every year at any time up to a depth of six inches for landscape plants.

A compensating feeding of ½ cup ammonium sulfate per bushel of mulch is needed for the bacteria in addition to the regular fertilizer. An alternate mulch feeding may be applied in the spring at the rate of 1 cup ammonium sulfate per 100 square feet.

• To assure good, healthy growth and heavy bud set, a basic fertilizer mixture which contains nitrogen as well as the other main essential elements, may be needed. It should be applied any time between October 1 and June 1. It is especially helpful if dug in when preparing beds as sites in the landscape for receiving plants. It is also useful for an annual application to established plants, keeping it away from the central stem and off the foliage. The rate of application is one quart of the mixture to 100 square feet.

Basic Mix
4 parts ammonium sulfate (NH 4 ) 2 SO 4 21-0-0  15 cents/lb
16 parts superphosphate 20% Ca 3 (PO 4 ) 2


8 parts triple superphosphate 46% Ca 3 (PO 4 ) 2 35 cents/lb
4 parts muriate of potash (potassium chloride) KCl 0-0-60 12 cents/lb
1 part calcium sulfate (gypsum) CaSO 4 13 cents/lb
1 part magnesium sulfate (Epsom salts) MgSO 4 46 cents/lb

Add ferrous sulfate yearly in the spring at the rate of one pint per 100 square feet to prevent iron chlorosis. If iron chlorosis is evident - yellowing of leaves with veins remaining green - mix 1 tablespoon per gallon ferrous sulfate and apply as foliar wash for a quick fix. The need is to restore the sap acidity. In magnesium chlorosis, the veins turn yellowish as well as the rest of the leaf, even to cream color, with browning of the leaf edge. Correct with magnesium sulfate at the rate of 2 tablespoons per gallon.

As a basic fertilizer, Clarence Rahn has found satisfaction with a fertilizer of 50% 10-5-5 and alfalfa meal. Alfalfa meal is cheaper than cottonseed meal and plants respond magnificently to its use.

• Spray when needed depending on season and area. Specific controls:

Lace bugs - Cygon 2E or Malathion 50% 2 teaspoon/gal or Orthene 1½ Tablespoon/gal or Sevin 2 T/gal or Isotox or Diazinon. Mid May and July. Deciduous azaleas are sensitive to Cygon. Look under new growth during first hot weather for signs of hatching or eating.

Canker worms & caterpillar - Dipel biological insecticide or Sevin.

Rhododendron Borer - Spray woody stems and branches late May and month later with Dursban or Lindane.

Black-Vine Weevils - Crescent-shaped notches in leaves are evidence. Orthene 1½ T. Repeat twice 7-10 day intervals. The few adults which remain over the winter, emerge in early May. Overwintering larvae emerge in early June and continue into July. Males are unknown.

Powdery Mildew - Late July with wettable sulfur 2 t/gal, Karathane, or Benlate. Mildew not important as it occurs late in season. Good feeding early in season minimizes.

Ovulinia Petal Blight - Hope for weather of low humidity when spraying is unnecessary and since blight is hard to control in high humidity. Use Daconil 2787 or Benlate. Triforine shows promise but not listed. Begin application when disease first appears and repeat at 7- to 10-day intervals during blooming period if weather is wet.

Phytophthora Root Rot - Best control is sub-surface drainage. In heavy soil plant on top of ground. Drenches to afford some prevention: Truban WP or Banrot 40% WP ½ to 1½ t/gal; Truban 25% EC 1 pint to 1 quart per square foot; Subdue 2 E systemic (ref. 4) is specific for pythium and phytophthora on a wide range of ornamental plants can be used as a soil drench or soil mix. A foliar spray may be used on azaleas.

Obscure Root Weevil - Adults feed on leaves with no definite pattern. Very rare in this area. Full coverage spray of Orthene 1½ T/gal to the foliage in late spring as soon as feeding is noticed - usually about May. Repeat every four weeks through September.

Applied late May, mid June, and late July.
Orthene 1½ T/gal.
Benlate 1½ t/gal.

1. Program at VF 11-19-82 — Lloyd Partain
2. Rhododendrons and Azaleas in the Philadelphia Area, ARS, Philadelphia area Chapters' Pamphlet, 1977, Hardening Off Rhododendrons and Azaleas — Lloyd Partain.
3. Pennsylvania Department of Agriculture, Regulatory Horticulture, No. 13, 1976, Lace Bugs; No. 4, 1975, Black Vine Weevil; No. 37, 1979, Rhododendron Borer; No.20, 1979, Petal Blight
4. Geiger News, Vol. 18, No. 2, April 1982.