How to Grow Kumquats Indoors

Potted kumquat tree near window indoors. Interior design
Liudmila Chernetska / Getty Images
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Kumquats are an interesting fruit. They appear similar to oranges but are much smaller, more sour, and often slightly oval rather than round. Kumquat trees (Citrus japonica) are relatively small flowering citrus trees. They grow in warm climates and also can be kept in containers, even indoors under the right conditions. The trees are slow growers with dense, glossy, dark green foliage. They bear small white flowers in the late spring to early summer that turn into the fruits. Some varieties will even flower and then produce 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.

Botanical Name Citrus japonica
Common Name Kumquat
Plant Type Fruit tree
Mature Size  6–12 ft. tall, 4–8 ft. wide (when grown indoors)
Sun Exposure Full 
Soil Type Loamy, moist, well-drained
Soil pH Acidic, neutral
Bloom Time Spring, summer
Flower Color White 
Hardiness Zones 9–10 (USDA)
Native Area Asia
Toxicity Nontoxic

Kumquat Care

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 their little fruits. The key is to make sure they get as much bright light as possible and to use a supplemental grow light if necessary. Also, avoid letting the plant sit in soggy soil. This is a common mistake new growers make, thinking their subtropical citrus trees like lots of water. While they do in fact like water, they can't tolerate sitting in oversaturated soil.

Indoors, these trees are protected from many pests and diseases that they could be exposed to outdoors. But still watch out for common household pests, including mealybugs and aphids. Use an insecticidal soap or horticultural oil on any pest issues. A healthy kumquat will yield dozens of fruits throughout the late summer and fall. Wait until the skins pick up a deep color and the fruits are slightly soft to the touch before harvesting.

Light

Kumquat trees thrive in full sun, meaning at least six hours of direct sunlight on most days. Indoors, place your plant's container by your brightest window. You might have to fit grow lights over it if you don't have a window that gets enough light. If possible, bring your plant outdoors to a balcony or patio in the summertime, so the kumquat tree can get plenty of natural light. Not getting enough light can hinder growth and fruit production.

Soil

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.

Water

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. 

Temperature and Humidity

Kumquat trees have a bit of cold tolerance, especially compared to some other fruit trees, and they can withstand temperatures down to freezing for brief periods. Indoors, aim to keep them in temperatures 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 what would happen outdoors at nighttime. 

Moreover, kumquat trees like humidity levels between 50% and 60%. To increase indoor humidity, especially during the winter months when the 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, ensuring that the bottom of the container isn’t actually touching the water.

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.

Kumquat Varieties

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 '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.
  • Citrus japonica 'Centennial Variegated': This is a compact cultivar with fruits that are striped in green and yellow.

Potting and Repotting Kumquats

When first potting your kumquat tree, choose a container that is slightly larger than your tree's root ball. A young kumquat tree will often fit in an 8-inch pot. Make sure the container has ample drainage holes. 

Plan to repot every few years as the roots outgrow the container, and choose just one container size up. 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 where it was previously situated. Ultimately the full indoor size of the tree will likely require a 16- to 20-inch container, depending on its variety and environmental conditions.