Greenhouses, in Portland, are the buildings for growing plants. From small sheds to huge buildings these green houses can vary largely in their structure. Greenhouses possess a structure with different types of covering materials, like- roof and walls made of glass or plastic sheets made of Polyethylene film or Polymethyl methacrylate. The incoming visible solar radiation easily enters these houses through the transparent glass walls, gets absorbed by the plants, soil & other elements inside the houses, warms up the interior air, which is retained inside the houses. Convection is the principle mechanism for heating up the green houses. The heated up structures & plants inside these houses release some of their thermal energy in the form of infra-red radiations, to which the glass walls are opaque. This adds to the convection effect to trap much of the heat inside the green houses and hence maintain a suitable atmosphere for growing plants especially when the outside temperature is not favorable for sustaining vegetation. There is a certain amount of unavoidable heat loss because of thermal conduction through building materials & glass walls. However, as a whole, the net energy inside the building rises & hence the temperature.
Greenhouses, in Portland, help protecting crops from extreme environmental conditions, shielding plants from dust storms and blizzards, and keeping out pests. Light, temperature & aeration controls inside the green houses allow improving crop production in marginal environments and thus contribute to a significant extent in the food supply to the countries of higher latitude. Apart from crop plants, Greenhouses in Portland are also used for growing flowers, vegetables, fruits, and tobacco plants. In most greenhouses, pollination is mediated either by the Bumblebees or by artificial means.