"A houseplant is a plant that is grown indoors in places such as residences and offices. Houseplants are commonly grown for decorative purposes, but studies have also shown them to have positive psychological effects. Houseplants also help with indoor air purification. Plants used in this fashion are most commonly, though not always, tropical or semi-tropical epiphytes, succulents or cacti.[citation needed] Houseplants need the correct moisture, light levels, soil mixture, temperature, and humidity. As well, houseplants need the proper fertilizer and correct-sized pots."