QBARS - v27n3 Lime In Rhododendron Culture?

Lime in Rhododendron Culture?
Otto Burchards, Oldenburg (Oldb.) Germany

From Rhododendron and Immergrune Laubgeholze, Jahrruch 1971
(Article translated from German by Dr. Herbert Heckenbleikner,
Quarterly Bulletin Contributing Editor)

In the literature about rhododendron and broad leaved evergreens, one always find the admonishment (Hinweis) "Rhododendrons do not like and cannot bear lime. For fertilizing one must use only lime-free fertilizer materials, so that only physiologically acid reacting fertilizers must be used. In watering the plants, a lime-free water (rainwater is best) should be used."

Aren't these statements too commonly observed when conclusions should be ruled by locales and areas where lime is present in the soil or may reach the plants through ground or sprinkle water? But in areas where there is very little lime in the earth, or none at all, lime has significance in the culture of these plants. Here one must consider the propagation of rhododendrons on rooting mediums and in containers, as is frequently done nowadays.

In order to discuss the problem of lime supply for rhododendron culture and plant propagation, the first thing to be considered is the significance of calcium in the plant's nutrition and the importance of lime in soil reaction.

The second thing to be considered is the question of the pH value, that is the soil condition of the natural environment of rhododendrons.

Also the manner of mixing the medium or soil in which the plants grow is important in breeding and propagation.

Sprinkler water has more than a little value in its quality and calcium content in the goal of producing good young plants.

In the enumeration of these factors one may find an indicated answer. This plant culture needs lime and it is indeed necessary here as much as it is for other plant growth. In each individual case it is necessary to determine if the plant has this element in its habitat, either in the ground or in the water, and in what quantity in order to determine if supplementing by the cultivator is necessary.

The pH Value of the Soil in the Native Habitat of Rhododendrons

Information about the necessity of lime fertilizer requires a laborious investigation of the pH value in relation to the calcium requirements. The pH value is determined by the quantity of H ions in the soil, the various acid groups, by the quantity of the plant roots and the activity of the soil microorganisms. The determination is by a potassium chloride solution (KCl).

According to their reaction the soils are arranged:

pH < 4 very strongly acid
pH 4 - 5 strongly acid
pH 5 - 6 moderately acid
pH 6 - 7 weakly acid
pH 7 - 8 weakly alkaline
pH > 8 moderately to strongly alkaline

For example, all heath and peat soils are very strongly acid. Here pH values can go as low as 2.8. The North German peat soils generally have a pH value between 2.8 and 3.2. Investigation of other peat soils could give other values but would probably be higher. For rhododendrons and most woody evergreens a pH value of 4.5 to 5.5 is indicated in the area of moderately acid. A decisive factor in the question of the pH value of a soil is its texture and its aeration, especially in humus rich soils. If the aeration is good then the pH value can also be lower. Repeated and massive use of peat, or the utilization of pure peat with rhododendrons in certain cases in which a natural lime availability can not be attained can result in a stunting of the plants. The pH values of the site of rhododendrons in their habitats gives us a disclosure about the optimal range that these plants require.

Lime in Plant Propagation

Peat is being used increasingly in propagation at nurseries. Compressed peat or peat moss are used according to the conditions of the business. Lately in the propagation of rhododendron cuttings and cuttings of other evergreen woods there is more use of peat moss and less of peat. The reason may be that in the drying process of peat, certain substances may be developed by oxidation. In peat moss there is little or no drying out and no residues to cause plant damage.

However, high temperatures in the propagation beds and high humidity, especially in the moist installations, favor the impact of peat moss substances unfavorable to the rooting process.

R. calendulaceum N. America 4.8-5.7
R. catawbiense N. America 4.6-5.7
R. ferrugineum Alps 3.8-5.3
R. lapponicum Scandinavia 6.0-8.0
R. maximum N. America 4.6-5.7
R. viscosum and canadense N. America 4.5-5.0
R. arborescens N. America 5.5-6.0
R. roseum N. America over 6.0

In the cutting beds the process is approximately as follows: About 6 to 7 weeks after the peat has been put in the propagation bed, the cuttings form roots. A light yellow or brown coloration of the roots develops. Later the roots die off and on as yet un-rooted cuttings a rot develops on the lower end. The symptoms show that unfavorable factors in the propagation medium are the cause.

In other aspects of plant propagation, peat is limed before use as a growth or propagation medium. Now the question is whether liming the cutting medium would be helpful since the pH value was around 3.0.

The first tests were made with Rhododendron 'Cunningham's White' and showed that the cuttings in limed peat yielded a higher percentage of rooting and the quality of roots and root balls was much better. The roots were white and of good appearance.

In a second investigation, various quantities of calcium carbonate were added to the peat in order to determine the optimum quantity of liming. It was observed that the addition of 0.5 grams of lime per liter of peat raised the pH about 0.5 points but that the rooting medium must have a pH value of between 4 and 4.5 in order to keep the harmful substances and acid bound to the peat.

This investigation was made with cuttings of a Rhododendron repens hybrid and Prunus laurocerasus 'Schipkaensis Macrophylla'. The investigations were carried out in a mist system.

Rooting of Cuttings of a Rhododendron repens Hybrid

Gram-Lime Well Percentage, heavily Not
per Liter rooted well rooted rooted
0 34 18 48
0.5 45 20 35
1.5 53 18 29
2.5 54 25 21


The cuttings of the research lot without lime formed only two or three roots, which branched and formed the root ball. The roots were easily broken off and formed mostly on the lateral cut surfaces. The lower portions of the cut surfaces were mostly brown from rotting. In the research lot with 0.5 grams of lime to each liter of peat the cut surfaces were mostly brown. The root and root ball formation was moderate. The research lot with 2.5 grams lime to each liter of peat yielded the highest percentage of rooting. The root balls were consistently somewhat smaller but the number of small fine roots was considerably greater.