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How To Fix Yellow Leaves on Kumquat Trees

My parents live in Ventura, California, and have a kumquat tree that normally does pretty well. However, it recently started getting a few yellow leaves. Since I had some spare time, I did some research to find out more and help them out. Here’s what I found can help kumquat trees with yellow and dropping leaves.

Yellow leaves on kumquat trees are typically caused by over-watering. However, other factors such as a lack of nutrients, root rot, and aphids can also cause this issue. The best way to prevent yellow leaves is to check that the soil is draining and inspect the leaves for aphids.

So, while there are several reasons why kumquat trees get yellow leaves, what can we do to fix it? Let’s take a closer look.

Over-Watering

Kumquat trees can get over-watered and develop yellow leaves fairly easily if they’re planted in clay soil or have potting soil that is collapsed. You can prevent over-watering by checking the soil’s drainage and only water when the soil is dry.

How to Fix

A good way to tell if the soil is well-draining is by digging a 1-foot by 1-foot hole nearby and filling it with water. From there, wait 1 hour. If the hole is still holding water, the soil needs to be amended.

To amend the soil and increase its drainage, you can mix in sand or perlite. These particles will help break up the clumps. Once the soil is well-draining, you can continue with adjusting the amount and frequency you water.

As a general rule, only water kumquat trees when their soil is dry. The best way to tell if the soil is dry is by pushing a finger into the soil, up to the second knuckle. Additionally, use 1-2 inches of both compost and mulch to improve water retention and prevent evaporation.

While knowing when to water is helpful, there are ways you can help your kumquat tree be more self-sufficient and water it less.

Compost improves the richness of the soil, which greatly increases its water retention. For example, each 1% increase in a soil’s organic matter can help hold 20,000 gallons more water per acre. It can also replace fertilizers (more on this later).

On the other hand, mulch is great for protecting the soil from evaporation and erosion. Mulch shields the tree’s roots and soil from drying out in the sun and wind. In turn, the rich soil (and its beneficial bacteria) provide extra water and nutrients for the kumquat tree.

When applying either mulch or compost, keep them at least 3 inches away from the kumquat tree’s trunk. Any closer and they can introduce mold.

A Lack of Nutrients

Unbalanced nutrients can cause kumquat trees to develop yellow and dropping leaves. To prevent and treat this condition, use a fertilizer that has double the nitrogen to phosphorus and potassium, such as one with a 6-3-3 NPK. Feed your kumquat tree 1-2 times per year (or every 1-2 months if using compost).

Poor nutrients can cause many issues for kumquat trees. Although, the most common are yellow and dropping leaves. While yellow leaves on kumquat trees are typically caused by low amounts of nitrogen, some other nutrients could be missing as well.

Fortunately, many fertilizers, including natural ones, contain sufficient primary nutrients (nitrogen, phosphorus, and potassium) along with enough trace nutrients.

How to Fix

Kumquat trees are fairly heavy feeders of nitrogen, so they need to be fed fertilizer about one to two times per year, or during their growing seasons.

For more information about kumquat tree fertilizer, you can visit my recommended citrus fertilizer page.

Alternatively, you can feed kumquat trees compost. Compost is easy to make and can have more useable nutrients than chemical fertilizers. If you decide to go with compost, apply 1-2 inches every 1 to 2 months during the growing season.

If you’d like to take this a step further, and make your own citrus fertilizer at home, check out my recent post: Craft the Perfect Homemade Fertilizer for Your Citrus Tree.

Keep in mind that a balanced soil pH is necessary for kumquat trees to stay healthy. A soil pH that’s either too acidic or alkaline will prevent the tree from absorbing nutrients from the soil properly.

ph scale couch to homestead

Generally, kumquat trees prefer a soil pH of 6.0 to 7.0. A good way to check the soil’s pH is by using either pH strips or a pH meter. To see what pH meter I use on my plants, you can visit my recommended citrus tools page.

Root Rot

Root rot is a fungal disease that can cause yellow and dropping leaves on kumquat trees. Typically, root rot is caused by poor drainage and overwatering. This disease will cause the roots to die off, which slowly kills the rest of the tree. Root rot can usually be fixed by repotting the soil.

How to Fix

Generally, root rot can only be fixed by repotting the tree with fresh soil. There are some other treatments, such as fungicides, but repotting is the most effective. By replacing the soil, it will remove most of the fungus and dry out the rest. As long as the kumquat tree doesn’t continue to sit in sopping wet soil, the remaining root rot will die off.

While I haven’t experienced root rot with a kumquat tree before, I have had it with my potted kaffir lime tree. After seeing the soil wasn’t draining and started to smell like a swamp, I found that the water had been stagnant for some time.

Because of this, the kaffir lime tree’s roots begin to rot and caused the leaves to yellow and drop. Fortunately, I caught it in time and repotting the tree, which saved it. It’s still doing great today and providing lots of limes (and leaves for Thai cooking)!

our dog Banjo with our Kaffir lime tree
Our dog Banjo with the Kaffir lime tree

Aphids

Aphids are small bugs that feed off leaves, causing the tree to slowly die. In the process, the kumquat tree’s leaves will begin to yellow and fall off. You can get rid of aphids by spraying them with water, spraying them with neem oil, or releasing ladybugs.

You can tell if your kumquat tree has aphids by checking the underside of the leaves. Aphids will show up as small dots, and come in a variety of colors such as yellow, white, and black.

How to Fix

My kaffir lime tree also had aphids at one point, and fortunately, I found that spraying them with a jet of water was enough to knock them off and prevent them from coming back. All I did was fit my thumb over the nozzle of a hose, and that created enough pressure to kick them off of the leaves.

If water doesn’t help, you can also spray the leaves with neem oil or release ladybugs (which are natural predators to aphids).

If you’re also having trouble getting your kumquat tree to fruit, make sure to check out my recent post: The Top 5 Reasons Why Your Kumquat Tree Isn’t Fruiting.