Outdoor cultivation is common in both rural and urban areas. Outdoor cultivators tend to grow indica-based strains because of its heavy yields, quick maturing time, and short stature. Some growers prefer sativa because of its clear-headed (cerebral) high, better response to sunlight, and lower odor emissions. Growers cultivate on their own property or practice guerrilla farming i.e. to plant cannabis in remote areas such as forest clearings or mountain cliffs which they rarely visit. However, such a method is prone to theft – so much so that some ingenious growers even attach pots to trees to decrease this possibility. Guerrilla growing has given birth to the activist movement Operation Overgrow, where the plant is grown with the explicit purpose of introducing the cannabis plant into the natural ecosystem. For outdoor cultivation, growers choose areas that receive twelve hours or more of sunlight a day. In the Northern Hemisphere, growers typically plant seeds in mid-April, late May, or early June to provide plants a full four to nine months of growth. Harvest is usually between mid-September and early October. In North America, northern locations are preferred (North Coast of California and British Columbia being particularly notable), but southern locations (such as Maui, Hawaii) are also known to be good producers. Where local laws do not permit growing cannabis, cultivators sometimes grow in forests or rugged and rural areas where the local population is unlikely to find the crop. Cannabis is also grown hidden by a crop that is taller, such as maize. This is reported by the United States government to be common in the midwestern states. Bamboo and elderberry are also used as camouflage companion plants. Some government agencies, including the Drug Enforcement Administration (DEA), have claimed that in State and National Parks people have been injured by cannabis farmers protecting their crops using booby traps; no arrests or convictions for this had been made . Cannabis can be grown indoors in a soil-like medium under artificial light, adding fertilizer when the plants are given water. Cultivating cannabis indoors is more complicated and expensive than growing outdoors, but it allows the cultivator complete control over the growing environment. Plants of any type can be grown faster indoors than out due to 24-hour light, additional atmospheric CO2, and controlled humidity which allows freer CO2 respiration. Plants can also be grown indoors through the use of hydroponics. To grow plants indoors, a growing medium (e.g. soil or growing substrate), water, nutrients, light and air need to be supplied to the plant (with the exception of aeroponic cultivation, in which case a growing medium is not required).