In most places of the world where gardeners have citrus trees, the weather is pleasant enough that citrus is grown commercially in large orchards. But for those of us living in the northern reaches of the world with less than accommodating climates for citrus, we need not be excluded from a year round supply of fresh citrus fruit.
As the cold winds and frosts force life into hibernation outside, you can be enjoying the benefits of a brightly coloured citrus tree inside. What better way to bring you out of the winter doldrums?
Use The Right Tree
Because the tree will be growing in a limited space select a dwarf variety of citrus fruit, one that will do well in a container.
At a nursery, pick a healthy looking tree between 2-3 years in age. Trees can be planted from seed, but it's not recommended. It can take a long time for a tree planted from seed to produce fruit.
Some good varieties for indoor citrus growing include: Calamondin oranges, Everhard orange, Otaheite oranges, Nippon oranges, Meyer lemons, Eureka lemons, Ponderosa lemons, Persian limes, Kaffir limes, and Rangpur limes.
Put The Tree In A Comfortable Home
Plant your tree in a 10-15 gallon (40-60 L) container with several holes in its base allowing for drainage; good drainage is paramount.
A layer of gravel at the bottom of the container will help even more with drainage and keeps the roots from drowning. Some landscaping cloth over the drainage holes will keep soil from washing out from watering, but it isn't necessary.
Soil pH should be slightly acidic, a pH of 6-7.
Keep the tree in temperatures between 55-85 degrees F (13-30 C) - around 65 F (18 C) is ideal. Avoid rapid shifts in temperature. Don't place near things like exterior doors, drafty windows, radiators, heaters, or ovens.
Allow The Tree As Much Sun As Possible
Citrus trees prefer 10-12 hours of sun a day, but winters in more northern regions don't often accommodate those needs. Citrus trees need a minimum of 5-8 hours a day. Place the trees by a south facing window, or, if you have one, in a sun room. Consider supplementing lighting with a grow light. Use a 40-watt, fluorescent, grow light placed just above the plant.
When the risk of frost is absolutely gone, the tree can be placed outside. Very gradually acclimate the tree to the conditions outdoors, it's been inside all winter. The process is the same as hardening off seedlings that have been started indoors. Begin by placing the tree outside for a few hours in a non-windy, semi-shaded spot. Day by day, increase the time spent outdoors and the time spent in direct sunlight. Don't rush the plant, without a slow transition the tree can suffer from shock and sunburned leaves. To bring the tree inside for winter, do the same process in reverse. Initiate the reverse transition long before the first frost date.
Keep It Humid
Citrus plants like moist air, but during winter most homes are around a dry 10% humidity. The best humidity range is around 50%. A citrus tree in an environment that isn't humid enough, will start to drop leaves.
Take a tray that will fit under your container and fill it with lots of small stones and a little water. As the water evaporates from the tray it will increase the humidity around the plant. The tray will refill with excess water from watering the tree.
Placing the tree near a humidifier or misting the leaves with a spray bottle regularly is other methods of providing humidity.
Check the top 2 inches (5cm) of the soil. When the top 2in (5cm) is moderately dry, water until excess liquid drips out the bottom of the container. You may need to water more frequently during summer months. Don't water too much, excess water in the soil can lead to lots of problems. Mulching will reduce evaporation, retain moisture, and provide some nutrition if organic material is used.
If using hard tap water, you may need to acidify a little the water to keep the proper pH in the soil. Use 1 Tablespoon (15mL) of vinegar per 1 gallon (4 litres) of water.
With the tree inside it will be away from pollinating insects. Many of these trees don't technically require pollination, but hand pollination can raise the chances of a good yield.
Gently rub the yellow tips of the stamens (anthers) with a cotton swab or paintbrush, then gently rub the pollen laden brush/swab over the tip of the central stalk (stigma). This is it, the plant will see to the rest.
Provide Enough Nutrients
Without enough nutrients, the plant won't have the resources to produce flowers and fruit. You can use a 1-1-1 plant fertilizer, compost teas, or amend the soil with some cured compost/manure. Feed the plant once every 3-4 weeks during summer and half as often during the winter.