How to grow rhododendrons in the north so they thrive

Rhododendrons are one of the representatives of acid-loving plants, with a pH of 5.5 to 6.5 being optimal. They prefer a cool, humid, well-ventilated environment, with a suitable growing temperature of 13°C to 25°C. During the growing season, relative humidity should be maintained at 60% to 70%, and as flowering approaches, relative humidity can reach around 80%.

Rhododendrons like light but cannot tolerate direct strong sunlight. In spring and autumn, increase shading appropriately, and in midsummer, achieve around 70% shading, and frequently spray water on the leaves to increase humidity and lower temperature. In summer, place rhododendrons outdoors for care to facilitate ventilation. Rhododendrons prefer light fertilizer but cannot tolerate concentrated fertilizer. During the growing season, apply thin fertilizer frequently, preferably farmyard manure, such as soybean cake water, horse-hoof water, and liquid fertilizer made by soaking and fermenting various vegetables, chicken feathers, fish innards and other miscellaneous materials. Rhododendrons have fibrous roots, which are fine and delicate, so heavy clay soil cannot be used. They prefer loose, well-drained, breathable acidic soil. In the north, mountain soil, larch leaf mold, or artificially prepared potting soil and other leaf mold can be used. After flowering, pruning is necessary to make the flower shape more beautiful and control the flowering period. The annual branches cut off can also be used for cuttings propagation.

If rhododendron leaves turn yellow on the upper surface and have small red mite spots on the underside, it is spider mite damage. In severe cases, it will cause leaf drop. It can be controlled by spraying with 40% dicofol at 1000 times dilution, once every seven days, three times consecutively. If the leaf surface is yellow but the veins remain green, it is an iron deficiency symptom. Spray ferrous sulfate solution about once every ten days, with a concentration not exceeding 5‰. Using vinegar and fermented rice washing water also has a certain effect. In addition, there are other symptoms such as: leaves lacking luster, red-brown dry tips gradually expanding, caused by direct strong sunlight, poor ventilation, and excessively high temperature. The environment can be improved to prevent and control this. If brown spot disease occurs, it can be controlled with chlorothalonil, carbendazim, copper hydroxide, thiophanate-methyl, etc. There are also cases where leaves turn yellow or remain very green, but with a gentle touch, the branches and leaves fall profusely and soon become completely bare. This is caused by light that is too dim and stuffy conditions; prevention and control can be carried out according to the situation when caused by weak light, excessively dry air, air pollution, etc. When rhododendrons lack water, they wilt and droop. New leaves that are pale and yellow are caused by long-term waterlogged roots. If leaves wilt but remain green and do not fall easily, this can also be caused by excessive water. Leaves that are uneven in size or have patchy black spots are mostly black spot disease, caused by pathogens, often due to a combination of excessive water, poor ventilation, and weak light leading to pathogen infection. The environment can be changed and chemical control can be used. For medication, refer to the treatment for brown spot disease.