Rhododendrons and other plants of the heather family

The general principles of the rhododendron
and other heather family plants cultivation

Young plants (rhododendrons, azaleas, kalmias and American Blueberry) bought on our farm need to be replanted from multipallets to bigger containers which should be of adequate size, i.e. 1.5 to 2 l.

The substrate on the basis of sour peat and with the following content is adequate:

70% of high, fibrous, sour (pH 3-4.5) peat with a low degree of mineralisation,
20% of shredded bark compost,
10% of perlite, sand or sandy clay.

The composition of the substrate for the cultivation of rhododendrons and other plants of the heather family described above has the right structure and provides the plants with the access of the air to their roots (it is as important a factor of the heather family plant growth as constant access of water in the substrate). The substrate with the composition mentioned above can be more easily watered after drying off (it does not shrink as much as peat itself and it does not come off the container walls), it is also a bit heavier. Plants after having reached a bigger size are not so easily blown over by the wind as the ones grown in pure peat. It is worth taking pains in preparing such a substrate, especially in small nurseries and at low experience in cultivating the heather family plants. In many big, commercial nurseries with an excellent irrigation and protection technology, the heather family plants are grown in the peat substrate without any additions and the cultivation results are excellent. This substrate (i.e. pure high peat) can be quite risky for the beginner producers, e.g. due to difficulties in watering after strong drying off of the root ball (especially when the nursery is watered manually) and because of the need for a more precise fertilisation of plants grown in peat.

The substrate for the cultivation of rhododendrons, azaleas and blueberries should be enriched with the addition of multi-ingredient fertiliser containing microelements, in a dose not higher than 1 kg of fertiliser per 1 m³ of the substrate, of course only while replanting the plants during the growth period.

Nowadays, the use of coated fertilisers, which gradually release their nutritional content during the plant growth, has become popular in the nursery cultivation. The results of their application are very good, the doses recommended by the producer should be used at their lower border (for most fertilisers the dose is not bigger than 2 g/l of the substrate in a container). While using the highest recommended dose of coated fertilisers (sometimes the producer recommends even 4-6 g/l of the substrate), especially when a high temperature maintains for a long time accelerating the release of nutritional ingredients into the substrate (this is a physical process whose speed depends on the temperature of the environment), the fluctuation of the substrate humidity should be monitored as the concentration of the soil solution may increase too much and plants may get damaged. This refers mainly to the short-lasting (3-4 months) fertilisers used by the nursery producers. During the period of high temperatures, after persistent rain (in the summer!), the period of time for which there is enough fertiliser for plants is shorter and the content of nutritional ingredients accessible to plants should be supplemented with irrigation with multi-ingredient fertiliser solutions in 1-1.2 g/l concentration (conuctivity of the medium should not exceed 1mS) or intraleaf fertilisation. The intraleaf fertilisation with commonly accessible solutions of fertilisers, of course containing some moistening agents, is extremely helpful in nourishing young plants.

We warn you against using fertilisers with unknown composition, whose producer or distributor does not give the content of nutritional ingredients on their packaging, their concentration and form. In shops you can easily buy fertilisers for amateurs, however, we rather advise you against using them.

Young plants bought in our nursery should rather be grown in an unheated greenhouse or a plastic tunnel which allow for making good use of the vigour and fast pace of growth of these plants. They can obviously be placed in the open air, but a little shading, especially at the beginning of their cultivation should be borne in mind. Plants cultivated outdoors always grow slightly slower than under covers. One should also remember to prepare plants for spending the winter by decreasing, from the beginning of August, the nitrogen fertilisation, strong ventilation of greenhouses and tunnels and providing them with full sunlight.

Many nursery producers shade rhododendrons in the spring and early summer, so that they get darker, “more attractive” leaves and grow more intensively, but the plants should have enough space and light to produce dense shape and be lignified enough in the autumn and prepared for the winter. It is best to quit shading them in July or August (plants should be uncovered gradually) and use fertilisers (with a low nitrogen content) speeding up their sprout lignification. In big nurseries shading of rhododendrons is not used in the spring and summer, and the plant material obtained in the sun is dense, perfectly lignified and has numerous flower buds. When you do not shade plants grown under covers, you should remember about intensive ventilation and water supply (plants should not be watered during heat waves, but at night or early in the morning).

The azalea and high blueberry should not be shaded!

In order to prevent the occurrence of diseases in the heather family plants one should avoid the following:

excessive fertilisation,
“flooding” them with water,
excessive increase of the number of plants in a nursery – free air movement in the vicinity of plants is fundamental, both in the summer and winter,
using uncertain phytosanitary substrates (e.g. mixing old substrate with clean peat, using compost which has not been disinfected),
using substrates of unknown composition (watch out salinity!) and origin, adding litter to substrates (risky practice).

You should remember about systematic ventilation of tunnels and greenhouses also during the time of rest of plants! All the alarming symptoms should be monitored and the treatments recommended in the heather protection programme should be implemented. In the case of noticing some alarming symptoms, do not hesitate to get in touch with our company.

It is best to winter young rhododendrons in unheatedplastic tunnels or greenhouses, which allows for minimizing the risk of damage caused by the cold, drying up wind and frost. Plants in closed spaces should be arranged in such a way that proper ventilation, irrigation and protection are possible. Having placed them for wintering, it is appropriate to spray them with antimycotic agents with the addition of an agent improving adhesion and absorption of the working liquid, e.g.: Topsin + Sadoplon + Euparen + Atpolan. One should absolutely remember about ventilating the places with rhododendrons, especially within the period of time when the root ball is frozen and the above ground part is not (warm days in the winter and early spring) – the plant transpirates and it cannot fill in the deficit of water through the root system, then watering may be necessary (even when the root ball is frozen), as well as ventilation to lower the temperature of leaves! Practice proves that considerable loss in winter is caused not because of the fact that plants get frostbitten, but because they wither!

In winter plants can also be kept outdoors. In big nurseries, plants growing in containers are often placed at an angle or horizontally, in lines, their leaves facing each other and covered with double green garden netting. It is done after a thorough irrigation of the substrate in containers and before the frost sets in. Covering with the netting prevents excessive drying up of plants by the dry winter wind. Moreover, the plants lying flat and covered with the netting are not damaged by snow.

Tadeusz Kusibab