Fragrant, zingy, and an essential culinary ingredient, citrus is not a crop to be left out of the hydroponic realm. While we may be more used to thinking of expansive orchards of spreading citrus trees grown in sun-drenched climates, with a little modification citrus can be a valuable and productive specimen for the indoor garden. By selecting high quality, grafted, and dwarf varieties of citrus, a thriving crop can be grown all year indoors, avoiding the limitations of cold outdoor environments and the risk of frost damage. As an added bonus, citrus flowers are highly fragrant and the glossy, deep-green leaves make an attractive display within any hydroponic system.

Citrus Types

While a wide range of citrus is grown commercially using hydroponic methods, these are largely long-term outdoor systems with drip irrigation. Some citrus such as Makrut lime (also called k-lime), grown largely for its fragrant leaves, are produced in greenhouses where heating is applied to extend the harvest season. With a smaller indoor garden, the objective is to maximize yields and fruit quality, while at the same time growing a healthy and attractive tree. To achieve this, investing in dwarf, grafted trees of known cultivars is the best option, as although citrus can be grown from seed, the characteristics of the resulting tree are highly unpredictable. The time to first flowering is often many years and fruit is generally inferior and lower yielding than that of named varieties on grafted plants. There are even some types of grafted trees with two or three different types of citrus grafted onto the one rootstock, making more efficient use of valuable growing space. Grafting tends to produce superior trees and crops as the rootstock provides additional vigor and disease resistance. Citrus trees are great specimens for restricted spaces as many varieties, such as lemons and limes, tend to fruit relatively young and while still quite small in stature. They can also be pruned and trimmed to restrain excessive vegetative growth and kept compact while at the same time, still be highly productive. If grown with a restricted root zone volume in limited sized pots, the height and spread of the tree can be minimized and citrus can even be grown as fruiting bonsai specimens.

Hydroponic Citrus Varieties

The most commonly grown citrus for hydroponic production are the smaller types of lemons and limes; however, there are some more unusual citrus types which offer a unique opportunity to grow a tree crop indoors. These include the tiny-fruited orange kumquat, the limequat, the Australian finger lime (Citrus austalasica) blood limes, the strangely shaped Buddha’s hand citrus, and the uniquely flavored calamansi. The Australian finger lime often called citrus caviar, is a relatively new specimen on the culinary scene which is currently being prized by chefs worldwide and one which is likely to become a valuable commercial hydroponic crop. Finger limes are a relative of citrus, native to Australia, but available from some citrus nurseries in the US. The fruit are elongated and finger-like, and are split open to reveal rounded lime green, tart, and juicy vesicles which resemble caviar. Finger limes require warm growing conditions and are frost sensitive outdoors, thus well suited to the year-round protected climate of an indoor garden.

For hydroponic production, suitable varieties of common citrus types include Meyer and Yen Ben lemons, Tahiti, Key and Mexican limes, mandarins, and sweet Washington navel oranges. Makrut lime is another small citrus tree ideally suited for indoor hydroponic production which produces a double crop of highly fragrant leaves which are finely sliced and used in Thai and other Asian dishes as well as small, knobbly green fruit which provide aromatic zest and juice. The advantage of Makrut lime is not only that the fresh leaves are far superior to the dried version, but the plant is often only grown for its valuable foliage and doesn’t necessarily need to flower and set fruit.

Start with Plants from a Good Nursery

For a new hydroponic planting, obtaining relatively young plants from a reputable nursery is a good place to start as these can then be regularly pruned to maintain a compact and attractive shape before flower buds start to form. Obtaining older, tall and sometimes straggly plants makes the process of height control much more difficult and productivity often suffers with this type of planting stock. Once plants are in place and well established, citrus should be pruned to remove the growing points of any dominant stems and promote the production of several side branches which will allow the plant to carry more fruit and remain compact.

Light, Nutrients, and Pots

Citrus, being fruiting plants, require a moderately high level of light to keep plants compact, prevent elongated growth, and produce sufficient assimilate (sugars) for high-quality fruit. A light level similar to that provided for fruiting tomato and capsicum plants is ideal, provided citrus trees are not overcrowded and kept pruned for maximum light interception. If light levels are low, citrus trees will continue to grow foliage, but often fail to flower or any sparse flowers that do open may not carry fruitlets through to maturity. Temperature optimums are also similar to many other hydroponic fruiting crops, although they vary depending on the type of citrus being grown. For optimal year-round growth, most citrus grows well within a temperature range of 75-86°F, however, will survive much cooler conditions down to 35°F for short periods of time.

Since most citrus fruits are a long-term crop and will continue to be productive for many years, growing containers of at least four gallons are recommended. Substrates such as coconut fiber or perlite are ideal, however citrus have been grown on a wide range of soilless mediums including gravel, grow rocks, peat, bark, stonewool, rice hull, and even in aeroponics. Good drainage is essential with citrus as oversaturated root zones can be prone to root rot, so the addition of some coarser material in the base of the growing container is always a good idea for long-term plants. Drip irrigation systems are most often utilized for nutrient delivery with standard vegetative and fruiting formulation applied during the different stages of citrus tree growth. A nutrient formation high in potassium with additional magnesium is essential during the fruit formation phase, as citrus fruit quality is related to potassium nutrition as well as environmental growing conditions.

Tips for Indoor Cultivation

Apart from good light and sufficient warmth, citrus benefit from some gentle air flow through the foliage. This helps remove any stale air and excess humidity and reduces the disease risk to the plants. One of the most vital aspects of growing fruiting citrus indoors is pollination which is essential for fruit set. Outdoors, bees and other insects carry out the pollen transfer role with great efficiency, however indoors this needs to be done manually. The process is relatively simple as citrus don’t need to be cross pollinated. A small paint brush can be used to collect the yellow pollen grains from within a citrus flower; these are then brushed off into the stigma (found at the top of the long column in the middle of the flower). The stigma becomes sticky when receptive to pollen, allowing the brushed-on pollen grains to adhere and stay in place. This process can be carried out a few times a week when citrus trees are in flower and is usually highly successful. It is normal for many types of citrus to produce masses of highly fragrant flowers, far more than the tree could ever support as fruit. For this reason, it’s not usual to see the excess flowers or tiny fruitlets drop from the tree – this is just a natural self pruning process which thins fruit to only those which the tree can support through to maturity. Young trees may only be able to carry a small handful of fruit in the first season and fruit loading will increase with the age and size of the tree.

Harvesting Hydroponic Citrus

Harvesting citrus fruit is relatively simple — its best to cut these from the tree rather than pull them off as this often snaps branches or causes fruit damage. Lemons and limes, once mature, can be left on the tree for a considerable length of time, often months, before harvest and this is often used as a way to store excess fruit until it’s required. However, retaining a high fruit load can delay flowering and the next crop of young fruit, so prompt removal is often a better option. Excess citrus fruit and even k-lime lime leaves can be stored in the freezer until required or processed into juice, jellies, marmalade, or syrups.

Whether it’s the new and exotic Australian finger lime or a common everyday lemon tree, hydroponic citrus fruits are a valuable addition to any indoor garden. Fragrant flowers, glossy deep green foliage, and bright decorative fruit make these ideal productive specimens well suited to hydroponic systems.

(For information on organic hydroponics, check out The How-To of Organic Hydroponics)