Nebraska has freezing winters and short summers. Grapefruit trees will not tolerate temperatures below or near freezing for more than a few days. They require warm temperatures and a long season. Growing grapefruit trees or citrus trees in a cold climate like Nebraska requires that the trees be grown inside. According to Charlie Ryrie the author of The Country Garden, hot houses or greenhouses are used in England with its cold climate to grow a variety of vegetables and fruits, including citrus.
Decide where the grapefruit tree should be located. It will require bright light--direct sunlight is best--for six hours a day. Eight hours are preferable. Place the pot support where the tree will go. Put the waterproof saucer on the pot support. Place the container on the saucer. The pot support allows the position of the tree to be more easily changed since it has wheels. The saucer keeps any drainage water from damaging flooring.
Fill the container with enough potting soil to bring the trunk of the tree up to the same level in the container as it was in the nursery pot. If the nursery pot is 12 inches high and the permanent container for the tree is 18 inches high, it will be necessary to put 6 inches of soil in the permanent container.
Remove the tree from its nursery container. If the plant is root bound, scratch the roots on the outside of the root ball with a knife to loosen them a bit. Place the tree in the permanent container. Fill the container with potting soil. Water until the soil is moist.
Turn the tree a quarter turn every week so it doesn't start leaning toward the window or light source.
Water when the top four inches of soil are dry. Dig down and feel the soil with your fingers. If it's still moist, wait a few days to water. If it's dry then water. How often you water depends on humidity and temperature. A greenhouse has higher humidity so it won't be necessary to water as often as if the tree were inside a house with dry central heating.
Fertilize with half strength water soluble plant food four times a year, three months apart. Mist the plant leaves once a week with distilled water. Distilled water doesn't have any salts or minerals.
Brush the blossoms of the grapefruit tree with a small paint or makeup brush to pollinate the plant and produce fruits.