Tillandsias are a group of subtropical and tropical plants that grow naturally in a wide range of conditions—as might be expected when you consider they can be found at altitudes from sea-level to around 4000 meters. Not the same plants, of course! Geographically they range from Texas down to Chile. Yet despite the diversity of natural habitats, it is truly amazing how many different species will grow together in cultivation.
Tillandsias have evolved to inhabit extremely harsh environments. Because of this, they are resistant to adverse conditions and some can endure several years of total drought and temperatures at or below freezing provided they are not wet too long. Luckily for us, this hardiness translates to a ready adaption to growing in unnatural places such as the frost-free garden, the house, greenhouse or shadehouse.
Tillandsias are often promoted as "air plants" which is true for most of them because they do take up their water and mineral requirements through their leaves, not their roots. But do not believe stories that they do not require watering. They are no different to any other living creature, they must maintain the water in their bodies or they die. They must be watered, but they do not tolerate being over watered. How much water they need depends on how dry the air is where they are growing (the "dryness" of air is not a simple relation to humidity. It is rather the capacity of the air to absorb moisture, which varies at various temperature/humidity combinations. For a discussion, see article "Water loss and the atmosphere" - above).
Bright filtered light is the general rule, and the higher the humidity of the air the higher light will be tolerated. Outdoors the silvery-leafed varieties can usually be grown in full sun, but in an unshaded greenhouse or close to unshaded glass in a sunny room or conservatory the same plant will quickly burn because the air dries out like an oven. In these conditions of extreme drying, and consequent moisture loss, tillandsias cannot get replacement water from their roots like a terrestrial plant, or draw on internal reserves like a succulent. Outdoors you will never need to water a tillandsia hanging in a tree if you live in a temperate climate. Indoors, the hotter and drier the air, the more you need to water.
In the home or greenhouse, if you keep the tillandsias in an area with bright indirect sunlight and sufficient air circulation to keep the air from drying out, they should maintain their moisture if you mist them once or twice a week or submerge them for a couple of hours every two weeks during the warmer months. In a very sunny spot indoors they may need daily misting or weekly soaking depending on which method you prefer. Symptoms of under-watering are usually inward-curving leaves or leaf-tip dieback. When over-watered, the plant will rot and fall apart. No resuscitation!
A note on holiday care: DO NOT go away for a month and leave your tillandsias soaking in the bath! Leave them in a cool, relatively shaded place with no watering. They will survive for months without watering this way.
Never water tillandsias when the temperature is under 10oC or when temperatures are falling. If you do, you run the risk of the plants staying wet too long and rotting. They do not like being wet and cold for extended periods (weeks at a time) without drying out regularly. In cooler autumn and early spring months watering should be cut right back unless you are supplying night heating, or live where the night temperatures do not go below 12oC.
In nature tillandsias have a very slow growth rate and survive on very little moisture and nutrient intake. With good light and warm temperatures during the growing season they can be encouraged to speed up their growth significantly with heavy watering and weak fertilising (no more than 50% of the recommended rate for houseplants). But be very careful if you live in an area with cool winter nights - you must cut back the watering and fertilising from late summer/early autumn so that the plant's slow down before the winter.
Raising tillandsias from seed is a lot of fun, but you do need patience!