Citrus trees such as oranges, lemons, and limes are generally quite easy to grow in greenhouse environments and offer a great way to enjoy the freshest possible fruit. Once you've had a greenhouse-grown orange, you'll never look at the ones in supermarket produce bins the same way again. Although lemons, oranges, and limes are the most popular types of fruit grown in home greenhouses, it's also possible to produce tangerines, clementines, kumquats, and grapefruits.
Selecting the Varieties
Dwarf varieties exist for all citrus trees and are far better suited for home greenhouse cultivation than their standard-sized counterparts. They don't take up nearly as much room, the fruit is the same size of that found on standard varieties, and they're not quite as prolific. Remember that agricultural researchers are coming with up with new varieties on a regular basis, so always ask a knowledgeable nursery professional for current recommendations concerning which types are best suited for your particular need and preferences.
Choosing the Container
Containers used in home greenhouse citrus production should be at least 10 gallons. Keep in mind that the bigger the pot, the larger the tree will eventually get. If you've got a particular favorite and have the room for it in your greenhouse, place it in a larger pot so its roots will have plenty of room to grow. The most important thing about the pot is that it's deep. Citrus trees tend to be top-heavy, and a deep pot keeps them balanced.
Heat and Water
Most citrus varieties will begin dropping their leaves when temperatures dip below 50 degrees Fahrenheit at night, so be sure to keep your greenhouse environment above that. Daytime temperatures should be above 70 degrees. Maintaining temperature levels appropriate for growing citrus can be expensive during the winter months, but you can help keep costs down by using a heavy greenhouse cover that will help prevent valuable heat from escaping through the roof.
All citrus trees require adequate watering, but many novice growers tend to overwater them. Because citrus doesn't like wet feet, use pots with plenty of drainage holes instead of just a central one in the middle of the pot. Placing pebbles at the bottom of the pot is a good way to provide added drainage, but this may make the pot heavier than you'd like if you plan on moving the tree to a different location in the greenhouse during its dormancy period. If this is the case, packing peanuts work just as well as pebbles, and they barely weigh anything at all.
Using a potting mix specifically developed for container cultivation of citrus trees also helps ensure that their root systems get enough drainage. Water the trees to the top of the pot when the soil has dried out. Curling leaves are a sign that citrus trees need to be watered as soon as possible.
Fertilizing Your Citrus Trees
Water soluble fertilizer with a high concentration of nitrogen works best for growing citrus trees. Working time-release fertilizer into the soil at planting time works better for most home greenhouse growers than mixing and adding liquid fertilizer throughout the season. No matter which route you decide to take, use a fertilizer specifically formulated for the type of citrus you are growing. Citrus trees grown in greenhouse conditions sometimes experience deficiencies of iron chelate, so be on the lookout for yellowing leaves and add iron chelate as needed.
Pests and Diseases
Whiteflies, aphids, leafminers, spider mites, and mealybugs have potential to affect citrus trees grown in greenhouses. However, these can easily be kept in check by using insecticidal soap. In the case of aphids, you can simply release beneficial insects such as ladybugs into the greenhouse environment. The best defense against pests and pathogens is to keep citrus trees as healthy as possible. To prevent fungal pathogens from becoming a problem, make sure your greenhouse has adequate air circulation and that your citrus trees aren't placed too close to other plants.
Most people prefer to move their citrus trees outdoors during summer and move them back into the greenhouse in fall. If you plan on doing this, it's a good idea to use insecticidal soap prior to bringing the tree inside to get rid of any insects or insect eggs that may be hanging on.
Knowing when to pick citrus can be tricky. Ripe citrus fruit is generally heavier than its non-ripe counterparts, brightly colored, and gives a bit when you press it with your fingers. Oranges won't continue to ripen once they're picked, so it's better to err on the side of caution if you aren't completely sure that they're ripe. If you plan on using them for juice oranges, keep in mind that the longer they remain on the tree, the sweeter and juicier they'll be when they're finally picked. Lemons, on the other hand, continue to ripen once they're off the vine while limes should be harvested when they're still green. As mentioned previously, limes are yellow when they're fully ripe, but they're so sour at that point that people prefer them when they haven't quite ripened.