How to Grow a Kumquat Tree Indoors

Kumquat is a smallish broadleaf evergreen fruit tree native to China and southeastern Asia. It is one of the better citrus trees for indoor growing, thanks to its small size and its habit of late flowering and a long fruit production season. Kumquat is a slow grower with dense, glossy, dark green foliage. It bears small white flowers in the late spring to early summer, producing fruits that gradually ripen well into winter. Some varieties will even flower and fruit twice a year. These trees are best planted in the early spring. Young trees might take a year or two after they're planted before they start to bear fruit.

Kumquat is not toxic to humans or pets, but gnawing on the wood may cause intestinal compaction problems in dogs and pet birds.

Can You Grow Kumquat Inside?

Kumquats have been in cultivation throughout Asia for centuries, and these small trees are perfectly adapted to container culture. When grown indoors, they likely won't reach their full size, but they still have the potential to produce lots of fruit. And there are several dwarf varieties that are ideal for indoor growing.

The key is to make sure a kumquat tree gets as much bright light as possible and to use a supplemental grow light if necessary. Also, avoid letting the plant sit in soggy soil. While they do in fact like water, they can't tolerate sitting in oversaturated soil.

The kumquat has been extensively hybridized over the years, resulting in several varieties that range in color from pale orange to deep red fruit. They include:

• Citrus japonica 'Centennial Variegated': This is a compact cultivar that is exceptionally good for indoor growing. It rarely exceeds 7 feet and can easily be kept pruned to 4 or 5 feet. The fruits are striped in green and yellow.
• Citrus japonica 'Nagami': This is the most popular type of kumquat with oval, deep orange fruit.
• Citrus japonica 'Meiwa': This variety is larger than 'Nagami' and has sweeter pulp and juice.
• Citrus japonica 'Marumi': This variety produces spherical fruits and tends to grow well in containers.
  

How to Grow a Kumquat Tree Indoors

Sunlight

Kumquat trees thrive in full sun, meaning they need at least six hours of direct sunlight on average. Even better is if you can give it eight to 10 hours of sun, though this can be difficult in the wintertime. Indoors, place your plant's container by your brightest window, and rotate the plant weekly. If possible, bring your potted kumquat outdoors to a balcony or patio in the summertime to rejuvenate it with plenty of natural light. Not getting enough light can hinder growth and fruit production.

Artificial Light

You may need to mount grow lights over your kumquat tree if you don't have sufficient sunlight in your home. Your grow light should be quite close to the top of the tree—about 12 inches—and should be left on for a full 12 hours per day. Choose grow lights specifically designed for citrus trees.

Temperature and Humidity

Kumquat trees have a bit of cold tolerance, especially compared to some other fruit trees, and they can even withstand brief frost. But strive to keep it in conditions between 55 and 85 degrees Fahrenheit. Make sure heaters and air conditioners aren’t blowing on them, as this can cause extreme temperature fluctuations, and protect them from drafts near doors and windows. It’s also ideal to decrease the room temperature by 5 to 10 degrees Fahrenheit at night to mimic outdoor conditions.

Kumquat trees like humidity levels between 50 and 60 percent but will do fairly well down to about 40 percent. To increase indoor humidity, especially during the winter months when forced-air heat can dry out the air, regularly mist the leaves of your tree. You also can place the container on a tray of pebbles filled with water, provided the bottom of the container isn’t actually touching the water. A cool-mist humidifier is another good way to optimize humidity levels.

Watering

Proper watering is essential for kumquat trees. The goal is to keep the soil lightly moist but not soggy. If the soil feels dry to the touch about 2 inches down, then it's time to water until you see the water run out the bottom of the pot. 

Air Circulation

Providing gentle air circulation will help your kumquat tree (or any citrus tree) flourish. A gentle breeze will help stabilize humidity levels and also assist with the circulation of carbon dioxide, which is necessary to the health of the tree. A gentle breeze also helps the plant self-pollinate.

If you have a forced-air heating system, this probably provides enough air circulation. Or, a small table fan set at a low speed somewhere in the room is usually sufficient.

Fertilizer

A high-quality fertilizer that's specially formulated for citrus trees will encourage healthy growth, flowering, and fruiting. Fertilize starting in the spring, following label instructions.

Pruning and Maintenance

Kumquat trees for indoor growing are generally dwarf varieties that don't need frequent trimming, but you should prune your plant back as necessary each spring to keep the tree small and manageable. Make your pruning cuts at a 45-degree angle just above a leaf node, which will encourage full branching.

Turn the tree every week to ensure that all sides get equal sun exposure.

Pollination

Outdoors, kumquat trees are self-pollinated by bees and other pollinators that transfer pollen from the male to the female flowers. This is necessary to fertilize the flowers so they can develop fruits. Indoors, you will likely need to hand-pollinate the flowers to duplicate the action of insects. It is quite easy to do with a small, dry paintbrush: Rotate the bristles around the inside of each flower, one after the other. This simple act will effectively transfer pollen from male to female flowers.

Container and Size

When first potting your kumquat tree, choose a container that is just slightly larger than your tree's root ball. A young kumquat tree will often fit in an 8-inch pot, which should last for three or four years before repotting is necessary. Make sure the container has ample drainage holes. 

Potting Soil and Drainage

Any quality potting mix that drains well will do for kumquat trees in containers. But a mix specially designed for citrus trees is ideal. You also can add a layer of gravel or small pebbles to the bottom of the container to improve drainage.

Potting and Repotting Kumquat

Kumquat trees bloom best if their roots are slightly confined in the pot. Plan to repot every few years when the roots have clearly outgrown the container; choose a container just one size up. Any type of material can work for a container—plastic, metal, wood, ceramic, or clay.

Gently loosen the tree's root ball from its old container, and replant it in fresh potting mix in the new container at the same depth as its previous pot. Ultimately, a fully mature indoor tree may require a 16- to 20-inch container, but a small plant will take several repottings over a decade or more before it requires such a large pot.

Moving a Kumquat Tree Outdoors for the Summer

It's a good idea to move a potted kumquat tree outdoors during warm weather to ensure it gets plenty of light. It will greatly appreciate getting eight to 10 hours of direct sunlight if you have the right spot. This can help bolster a tree that hasn't been getting enough indoor sunlight.

Considerations

Kumquat trees are warm-weather plants that don't respond well to extended temperatures below 50 degrees Fahrenheit. It won't be harmed by an occasional cool night, but make sure average nighttime temperatures are sufficiently warm before you bring your tree outdoors for the summer.

When to Bring a Kumquat Tree Back Inside

When nighttime outdoor temperatures begin to routinely dip towards 50 degrees or lower, it's time to bring your kumquat tree back indoors to the brightest, sunniest spot you can find. Make sure it is moved indoors before the first hard frost.

It's a good idea to spray your tree for pests before bringing it back indoors.