the dry indoor air and inconsistent watering many houseplants are subject to. Often, houseplants from tropical areas are understory plants, and because they grown in shady conditions naturally, they are often able to thrive in lower-light conditions. Houseplants are typically grown in specialized soilless mixtures called potting compost (in the UK), potting mix, or potting soil. Most potting mixes contain a combination of peat or coir and vermiculite or perlite. Keeping plants consistently too wet ("overwatering") leads to the roots sitting in water, which often leads to root rot. Root rot is the most common cause of death for houseplants but keeping houseplants too dry ("under-watering") can also be detrimental. Plants require nitrate, phosphate, and potassium to survive, as well as micronutrients including boron, zinc, manganese, iron, copper, molybdenum, and chlorine. Houseplants do not have access to these nutrients unless they are fertilized regularly. House plants are generally planted in pots that have drainage holes, to reduce the likelihood of over-watering and standing water. Pots are typically broken down into two groups: porous and non-porous. Porous pots (usually terracotta) provide better aeration as air passes laterally through the sides of the pot. Non-porous pots such as glazed or plastic pots tend to hold moisture longer and restrict airflow. Houseplants experience a range of pests. Fungus gnats, spider mites, mealybugs, thrips and scale are common pests. Alternative growing methods Houseplants are also grown in a variety of media other than potting mix, often in a hydroponic or semi-hydroponic system. This may overlap with aquascaping. Materials like sand, gravel, brick, expanded clay aggregate and styrofoam may be employed. Some epiphytic plants may be grown mounted, either with their roots in potting mix and their stems attached to supports, or with their roots wrapped in sphagnum moss and attached to a vertical surface with wire. This can also overlap with the practice of kokedama. Production Houseplants are obtained either by collecting wild specimens of plants, or by growing them in greenhouses or commercial nurseries. Wild collection of plants for the houseplant trade is a major threat to many species, especially plants with limited native ranges. Plants which are particularly threatened in this way are cacti, succulents, and carnivorous plants, especially Venus flytraps. Plants grown for commercial production may be produced from seed, by using traditional propagation techniques, or through tissue culture. In 2002, the wholesale value of foliage plants grown in the United