In relation to some intensive methods of the heather family cultivation implemented by many nursery producers, often in plastic tunnels with the application of high fertilisation to obtain the commercial material as quickly as possible (lowering the costs); I would like to draw your attention to several aspects connected with irrigation and fertilisation. The quality of water used for watering the plants of the heather family greatly affects their successful cultivation. Each type of natural water contains dissolved mineral salts. By dissolving mineral fertilisers in it we increase the total quantity of mineral salts dissolved in it. Irrigation with water containing too many dissolved mineral salts causes their gradual concentration in the substrate, stops the root growth, makes the tops of the youngest leaves wither, causes the appearance of the symptoms of physiological drought (the loss of turgor, among others, despite the accessibility of water), and dying out of the whole root system, which in consequence leads to the loss of the plant. Normally, right after the damage to the root system, leaves and sprouts get covered by some changes resembling to a great extent symptoms of the diseases caused by mycotic pathogens, which most often is wrongly recognized as the cause of the withering of a plant.
What should the water for irrigating and fertilising the heather family plants be like? In general, soft water is the best, with low salinity, slightly sour, rich in oxygen and not containing ions of heavy metals and chlorine. Rain water fulfils such requirements in ecologically clean areas. Whether the water accessible in the given area is suitable for watering the heather family plants can be stated by simple tests. You can have these tests done by a chemical and agricultural centre or a regional disease control centre. It is necessary to determine a few parameters characterizing water, which will allow to assess its usefulness for watering the heather family plants. The following belong to the most important parameters determining the properties of water: salinity (expressed in EC units), hardness (expressed, e.g. in °dH – German degrees of water hardness) and reaction (expressed by the pH value).
The salinity of water is the sum of the content of all mineral salts dissolved in it. Water from the water supply systems, ponds or wells always contains some dissolved mineral salts. Their quantity and quality decide about the usefulness of water for gardening purposes. The geological conditions affect the type of mineral salts dissolved in water.
There is a relation between measurable electrical conductivity of water and the quantity of salts dissolved in it. Distilled water or rainwater contains few mineral salts and they have small conductivity, while the water containing a lot of dissolved mineral salts has big conductivity. In practice, the EC (electrical conductivity) units are used to determine the salinity of water. Siemens (S) is a unit of electrical conductivity of solutions (in this case of water which is a natural solution of mineral salts). 1 S = 1,000 mS (milliSiemens) = 1,000,000 µS (microSiemens).
In gardening, the unit of 1 EC is used and it equals 1 mS (miliSiemens). The conductivity of water supplied to a nursery usually equals 0.1-1.0 EC. For comparison, distilled water has EC close to 0 (between ten and twenty µS – microSiemens). Conductometers are used for measuring the water conductivity, which is the amount of salts dissolved in it. Simple, battery operated conductometers, available in Poland at the price of several hundred zloty, are sufficient for gardening purposes, but they should rather be equipped with a system compensating for the influence of temperature on the conductivity of solutions, which allows to read off the real value of EC). The use of a conductometer does not come down only to assessing the usefulness of water for irrigation. It also allows to measure the conductivity of the fertiliser solution (medium) used for fertilising plants and it also enables the assessment of the substrate salinity in containers and the nursery, which makes taking a decision about fertilising easier. If the water accessible has EC <0.1 (i.e. < 0.1 mS), it can be recognised as perfect for nursery purposes, for plants sensitive to salinity and for cultivation in small containers. If the EC value does not exceed 0.5, it will still be good for irrigating more resistant varieties in bigger containers. Water with EC > 0.5 is completely unsuitable for irrigating cultivations in small containers, especially under coverings, and it needs to be treated, i.e. demineralised. When the temperature of the environment is high, the substrate in small containers dries quickly and it needs frequent watering. The mineral salts concentration in the substrate is quick, and the concentration of the soil solution becomes so high that roots may be damaged. One should also bear in mind that many commonly used artificial fertiliser pellets with inhibited effect release their nutritional ingredients into the soil solution proportionally to the temperature. Strongly salinated water (EC > 0.5) can be used for watering plants in bigger containers, placed in a container nursery, where the rain, from time to time, leaches the excess of mineral salts out of the substrate. To sum up, the heather family plants require water with a low content of mineral salts.
The conductivity of fertiliser solutions for sensitive varieties (especially in small containers) should not exceed 0.6 mS, the remaining varieties (more resistant) can be watered with the fertiliser solutions with EC < 1.
The hardness of water water is directly connected with its salinity. Calcium, magnesium and sodium salts (mainly carbonates and sulphates), dissolved in water, limit its usefulness for irrigation. The hardness of water is measured, among others, in °dH (German degrees). 1°dH corresponds to the content of 10 mg of calcium oxide in 1 l of water. Calcium and magnesium carbonates and bicarbonates cause the so called temporary hardness which can be removed by acidifying or boiling water (residues at the bottom and on the walls of a vessel). Calcium and magnesium sulphates cause hardness irremovable by boiling, the so called permanent hardness. Soft water with 0-10°dH hardness is useful for nursery purposes. If its hardness is higher and plants are cultivated in small containers and under coverings, then such water may need treatment. Hard water has alkaline reaction (pH > 7) and needs acidifying. Long lasting irrigation with alkaline water (at the same time most often hard and often salinated) increases the reaction of the substrate which results in disorders in absorbing nutritional ingredients (mainly microelements) and worsens both the growth and looks of plants. In practice nursery producers usually add acids by means of a proportional dispenser and constantly monitor the salinity and reaction of water (pH).
The reaction of water used for watering should be slightly acidic.
If we irrigate with harder and slightly salinated water, then we should not allow excessive drying off of the substrate in containers since it leads to excessive increase of concentration of the soil solution. Constant access of the air is important for a healthy development of the root system of the heather family plants. This is provided by loose substrates, not compressed hard while planting. In our nursing practice peat is still the basic ingredient of substrates. It should be light, sour and with a low level of mineralisation (then it has low solidity and good structure). All additions e.g. bark compost, substrate made from the heather family plants or natural substrates ought to be absolutely examined, paying special attention to salinity and reaction. After many years, thanks to the information on cultivation of varieties of different plants, made available by many nursery producers, it was possible to make a list of species and varieties which are especially sensitive to higher salinity and the excess of water.
Below you can find a list of species and varieties sensitive to high salinity of the substrate and bad water quality used for irrigation:
• Broad-leaved Kalmia and its varieties – apart from the water with low salinity, hardness and the right pH, it additionally requires an absolutely drier position and a more permeable substrate,
• The Rh. repens rhododendrons, i.e.: ‘Scarlet Wonder’, ‘Elvira’, ‘Bad Eilsen’ and others,
• Multiflower varieties of rhododendrons, such as: ‘Hachmann’s Charmant®’, ‘Lachsgold’, ‘Karibia®’.
Special care should be maintained when the above mentioned species and varieties are young and grow in small containers. It is very important that fertilising and irrigation were done while observing non-extendible norms of the water quality and salinity of the medium, and the substrate in containers was not overheated (shading, light capillary mats). In the last years we have experienced short periods of extremely high temperatures during the growth of plants in late spring and in the summer. The need for frequent irrigation in such a period of time may cause the occurrence of symptoms of excessive salinity even then, when in previous years in a similar season of the year and at the same technology of cultivation they did not occur at all. That is why it is very purposeful to exactly determine the characteristics of the water used in a nursery.