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Camellias are easy to cultivate and suffer from only few minor deseases. Where these plants grow naturally in open ground, there is very little care necessary since most growing conditions are given by itself or cannot be changed. In more harsher regions, camellias are cultivated in containers and partially under glass, and there we need more care and attention. Cultural rules should support the plant in their natural needs and behaviour. Thus it is helpful to find out and describe these parameters, in order to assist the plant development.
Camellias have only a short vegetative phase of about 4 weeks in spring. The rest of the year they stay dormant (summer dormancy and deeper winter dormancy) where they induce and mature their flower buds. Sometimes a second, unwanted shoot occurs later in summer, but mostly will not result in flowers because the warmth requirement is not given in these late shoots. In young plants the second shoot is encouraged.
Camellias need warm summers and cold winters The temperature during summer should be above 20°C (>70°F) for several weeks, even at night. This period insures the induction of plenty flower buds. Flower buds can be discriminated from vegetative buds by their form, they are round and thick. All of the flower elements are build at this time, if you dissect the bud you will detect all stamens, petioles and petals. After this the plant can be kept cold
Sometimes you read that camellias need to have a sunny autumn. This is only true if the plant did not have enough temperatures at summer. Outdoor plants need, in addition, maturation of their shoots to be hardier in winter.
The flower will develop later in high quality, only if a certain mixture of plant hormones is present. This is granted by a cold treatment of several weeks ( below 10°C or 50°F); where degrees below -3°C (25°F) must be avoided under all circumstances with potted plants. Normally camellias can be put outside during spring and summer, and must be brought back to the glasshouse in autumn. If, however, the summer stays cool and wet, it might be better to repose the plants in the glasshouse in June, for the temperatures are higher there.
Camellias need light during summer The number of induced flowers depends positively not only on temperature, but also on high light intensity and long daylenght. Direct sun is tolerated by some cultivars, however, especially white flowering plants do not develop dark green leaves. In any case caution is necessary if young leaves have developed under glass, they are very easily sunburned.
At lower temperatures (below 10°C or 50°F), camellias need only little light and below 5°C (40°F) almost none. Thus, during winter, camellias may be stored in the garage or a cool basement. Although not important, short daylenght in winter is not necessary, the plants prosper and flower also with long daylenght all year.
Camellia roots are water and salt sensitive Camellia plants, like others, demand moist soil. They are, however, under considerable stress if their roots are constantly under water. Thus it is of paramount importance that the potting ground must have only little water retention and drainage must be sufficient. If the ground contains too much loam, the particles will gather round the roots, and especially young plants will not prosper and show only yellow leaves ( small pots are anyhow difficult to control in their moisture content ). If you recognize earth-worms ( Lumbricus terrestris)in your camellia pot, you likely have this situation. However, during summer the plants cannot be overwatered if the ground is porous. This is quite different in winter with cold temperatures: if the roots stay wet, the plant will eventually shed all her leaves within a few days. ( If this happened, hold the plant at about 10°C (50°F) and put under the pot a flat aquarium heater of about 12 watt in order to dry the roots ).
Avery good (and old) potting substrate is 'heather ground' (Heideerde), which is not found in heaths but under pines ( Pinus sylvaticus ), especially where bilberry ( Vaccinium )is found. Just take the 5 cm decayed layer and pass it through a sieve. This soil is very useful but needs frequent watering since it is very porous. Thus it can be mixed ( 1:1 ) with packed garden soil for cuttings and seedlings ( low salt content ). Other constituents may be decayed wood or leaves or peat, all for humus. For a very good discussion of potting substrate see the posted article of M. B. Thomas in the New Zealand Camellia Bulletin # 124 (see under journals).
The pH value is best around pH 5. Rainwater is best for potted plants. If you have to use tap water, be careful: It is usually alkaline, especially if it is soft (pH8!). Thus it is good practice to acidify your water with peat or an organic or anorganic acid. I use a stock solution of 10% sulfuric acid, which is not agressive. I usually add 1 to 3 ml of stock solution to 10 Liters of water. ( If you decide to prepare your own stock solution: be careful! Concentrated H2SO4 is extremely harmful. Pour acid to water only in small portions, never vice versa, and let it cool down between, it may become so hot that glass breakes!) If you recognize brown/grey remnants on the soil surface or pot rim, your water is too alkaline.
Salt injury, which occurs easily, is indicated by camellias in that young leaves get brown or black points, borders or spots. Water the pot thoroughly or better repot in more suitable ground. Be careful with fertilizers.
Camellias flower best at low temperatures
These ornamental plants have the ability to develop their flowers at temperatures below 10°C (50°F), and their quality as well as their duration ist best there. If a budded plant is transfered from cold to constant temperatures above 15°C (60°F), it certainly drops its flower buds ( even in moist atmosphere ). Important seems to be the night temperature, day temperatures rising up to 30°C (90°F) are tolerated by most cultivars without negative signs during the flowering period.
In Germany, camellias are mostly advertised as room plants. This is the main reason for failure and disappointment, since the plant will drop flower buds if not given appropiate cold treatment, or will even die if brought in a heated room.
These were the essential rules. If followed, it is very likely that you are successful. Other, less important but helpful rules will come soon.
These rules were compiled according to my limited experience and to literature research. Other, more experienced specialists may have different opinions and are invited to contradict and protest, hopefully. Only this way we come closer to truth. So please respond!

Basic Cultivation Parameters by K. Peper.

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