Wednesday, November 3, 2021

Growing Indoor Citrus - 22 Things You Should Know


[Originally published 22 November 2019; updated 4 November 2021]

In our bedroom it is citrus season.

It is cold outside, but our indoor citrus trees are flourishing. They will only move to our small Brooklyn terrace again in late April, when overnight temperatures stay consistently above 50'F. These trees have given me so much pleasure, and have prompted so many questions over the years, that I thought I'd share some of my citrus growing tips. There is a lot to tell. Go fetch a cup of coffee. And maybe a sandwich. 


Whenever I post a picture of the trees, or fruit, or a cocktail made with fresh-squeezed juice on Instagram, I am asked questions: 

"What is wrong with my citrus tree?" 
"Why did my Meyer lemon die?" 
"Why are the leaves on my tree's leaves yellow?" 
"Why did all my citrus leaves drop off?"

I hear you. But it is impossible to give good answers without knowing exactly how those trees have been cared for. So to help stressed citrus parents and to troubleshoot common indoor citrus problems (and to encourage more people to grow citrus trees indoors), I have put together a list of indoor citrus care tips and problems, based on my own experiences, and with some expert advice gleaned from professionals.

You may recognize your tree's issues, or a care factor you may have overlooked, or not known about. You won't need to know this all at once (except No. 1) but bookmark it for when you have questions! Because you will have questions.


I bought my first Thai limes (Citrus hystrix) in 2016 (see the skinny, above) never dreaming they would make fruit. I would have been happy with their famous, double leaves, to use in curries and drinks.

(By the way, if you wonder why I call them Thai limes, read this post about the k-word).


But produce limes they did. I have been giving them away. Growing an unusual fruit that is hard and expensive to source is a wonderful experience. 

When we moved, one of the criteria for our next apartment was sun for the limes: South-facing indoor light for the overwintering trees. Luckily, we found it.


A Meyer lemon, a yuzu (Citrus x juno), and a petite Australian lime (Citrus australasica) joined the jungle. (And that is a bay tree in the foreground. But let's stay focused.)


Growing citrus is easier than most of us think. But it does require regular attention. And that's a big, big caveat. Attention requires focus and time. Not everyone can afford those.


So let's begin with the most common problem. By far:

1. Overwatering Your Citrus Tree Will Kill It

It's natural to want to water your tree. But citrus trees will die if they are overwatered. They must dry out in-between deep waterings. Only water again when the pot is almost dry.

Let's explore that: Outdoors, excess water pouring from a pot's drainage holes can just run away (I never keep my outdoor pots in saucers). Indoors, the runoff is trapped in a saucer. Sitting in a puddle of saucer-water for longer than 12 hours is bad for your tree. If the soil stays moist the trees' roots will rot slowly and the tree will die. If you spot the problem in time, it can be corrected, but it will take months.

Signs of Overwatering:

If the outside of a terra cotta pot is always dark and damp or green at the base.
If the the soil stays dark and moist to the touch every day, with no change.
If water stays standing in the saucer beyond 12 hours.
The mature leaves are drooping (this takes longer to show up).
The leaves gradually turn yellow all over and drop.
Little bugs like fruit flies hover everywhere: these are fungus gnats and are an indication that the pots are staying moist enough to grow the fungus they feed on. A large infestation of their larvae can damage roots.
The roots, if you remove the tree from the pot, disintegrate in your fingers. This is an emergency. See No. 5!


2. How to Water an Indoor Citrus Tree

Make sure you have a saucer under the pot! Don't laugh. Now drench the pot. This means: give your citrus tree enough water that it begins to freely from the drainage hole/s. If no water runs out you have not watered enough. Add a little more. Take note of how much water you used, for future reference. I use a large measuring jug. My mature Thai limes drink 8 - 10 cups each. 

Trees need more water when they have fruit. But keeping my limes on their fruit-watering schedule made me overwater them by accident after the fruit had been harvested. Result? Root rot and an emergency repotting. So - pay attention. 

Nancy Lingnan handles customer support at Lemon Citrus Tree where I bought my Thai limes and Meyer lemon. She suggests keeping trees on a stand above a substantial plastic saucer that can accommodate about a gallon of runoff. The water just pours out into the saucer without drowning the tree by letting it stand in a pool.


I don't have stands and giant saucers because of space constraints. Instead, I let a small amount drained water remain in the saucer for up to 12 hours (thirsty trees will absorb this water again). After 12 hours, I suck up any excess water with a turkey baster (above: an unlikely indoor gardening supertool!). On no account should the water remain longer. 


Only water again when the pot is close to dry

In terms of touch and feel, the top inch or two of soil will transition from dark and moist, to lighter and dry. Time to water. A moisture meter can help you (see Tip No. 5) - I water again when it registers 2 - 3 (Nancy waters again at 4).

With time and experience you will learn how much water is ideal for your own citrus trees. Different rooms, indoor temperatures, pot sizes, and tree sizes will make it a very individual thing. For example, my Thai limes (almost five feet high), in 14-inch pots, receive 10 cups of water every 5 days when they have fruit on the branches. But with the fruit harvested from the trees I water them every 10 days.

If you suspect overwatering, stop watering. Buy a moisture meter. Allow the soil to dry out. Water only once the pot is almost entirely dry again.


3. Water Temperature for Citrus Trees

Cold winter tap water is a shock for subtropical and tropical citrus trees indoors. If your tap runs icy, take the edge off with a dash of hot water. The water should be roughly the temperature of the room they are in. You could also leave the water overnight in a watering can to reach room temperature.


4. Poor Drainage Kills Citrus Trees

Citrus trees require outstanding drainage. Water must flow right through the pot and out. Soggy bottoms will kill them. Your pots need drainage holes, and a very well-draining soil mixture. 

Potting myth: You don't need shards or gravel in the bottom. They do more harm than good.

I mix good potting soil (I like Black Gold) with large handfuls of untreated shredded bark. My mix is 50:50 potting soil and orchid bark.

If you have plastic growing pots set inside an ornamental, closed pot, it is imperative that you never allow the citrus to stand in water inside the ornamental pot. After 12 hours, drain it (see No. 1). More work for you, but essential.

Signs of poor drainage: 

Damp pot bottoms.
Green algae-like growth on the outside of the pot.
Constantly moist soil.
Fungus gnats in the room.
Yellow leaves, drooping leaves, dropping of green leaves.


5. Root Rot Requires Immediate Intervention

Root rot is what it sounds like. But it is a collective term that can have several causes. Possible culprits include serial overwatering, poor drainage, standing in wet saucers, or attack by a fungus (unrelated to overwatering). The roots of the citrus tree have begun to rot. You can only know this for sure by inspecting the roots. Lay the pot on its side, gently slide the rootball out. 

Look at the smaller roots closely and feel them in your fingers. If they break very easily, at a touch, or if there is a mass of worm-like broken roots in the pot, or if you see white threads protruding from the darker root sheaths (I call them the white threads of death so yes, this has happened to me), root rot is at work. Act fast.

Trim off as many damaged roots as possible. Wash the pot thoroughly with soap. Dry it. Add new, fresh potting soil and bark (50:50), and repot. You will have to water the tree as soon as it is repotted, but then resist the urge to water again until that soil is almost dry. Cross your fingers. And wait. Very heavy-duty fungicides (usually only available in industrial quantities) can treat some root rot, but they can be very toxic.

Signs of root rot:

Small branches and twigs die back (they turn brown and dry)
General loss of vigor.
Yellowing, dropping leaves.


6. Underwatering Citrus Trees

Perversely, underwatering is less common and also less serious than overwatering. Chronic underwatering will kill your citrus trees, but the signs are much easier to spot and to fix.

Citrus trees need infrequent but deep watering, so a sprinkle on the surface will not help them. Water the citrus trees deeply till water begins to run from the drainage holes. Keep a watering diary if you are prone to forgetting.

Signs of Under-Watering:

New, tender growth droops, leaves hang limp.
If there is fruit it feels flaccid when squeezed (...ahem) - it should feel very firm.
Soil pulls away from the sides of the pot.
When you water, the water pools clearly on top of the soil for a while before sinking in, then rushes right through the pot and out (if it does, allow the tree to absorb that runoff from the saucer, then water again).


7. Buy a Moisture Meter

To help take the stressful guesswork out of watering citrus, buy a moisture meter. Insert it fully into the growing medium. In larger pots it is helpful to take two to three readings in different spots. I water again when all three readings in a pot reach 3. (After the drenching it should read 10.  If it does not, the water has flowed out too fast because the pot was too dry.) Allow the standing water to be absorbed.


8. Fertilize Citrus Trees Every Month

This is not what most resources will tell you. It's not even what the fertilizer bags and boxes tell you. But professional growers feed often. Indoor citrus trees in pots need more frequent applications of fertilizer than citrus outdoors and in-ground.

In milder winters where citrus trees remain outdoors they slow down in colder temperatures and feed less. But protected indoors, the trees actively grow. In winter I see new shoots,  a lot of flowers, and the setting of new little fruit. The energy required depletes the plant, which requires nutrition to stay healthy. Every time you water, food is being made available to your tree, but is also washing out of the soil.

I fertilize organically and feed my citrus trees with Citrus Tone, following the dosage instructions for pot diameter on the bag. Citrus Tone also contains beneficial microbes to help make nutrients available to the plant. Very recently I added Down to Earth's Citrus Mix, another organic fertilizer that also includes the micro nutrient zinc - I had a possible, mild zinc deficiency in one of my Thai limes. It's an experiment. I will alternate between the two. Citrus Tone has kept my trees happy for years.

To apply granular fertilizers like these, gently scratch the fertilizer into the surface soil and water immediately. A light mold can form a few days after fertilizing (with these products); it is nothing to worry about, just scratch it back in.


9. Healthy Citrus Leaves should be a Uniform, Rich Green

Signs of nutrient deficiency are diverse and hard to decipher, but each tells a different story - the details are important when reaching out to an expert for advice:

Very pale yellow leaves (could be an iron deficiency).
Blotchy yellow and then completely yellow older leaves (possibly nitrogen deficiency).
New and maturing leaves with green veins with yellow blotches between (could be a zinc deficiency).
An inverted green V-shape surrounded by yellowing (magnesium deficiency)


10. Use a Citrus Nutritional Spray to Correct an Emergency Deficiency

In a nutrient-emergency I have used a citrus nutritional spray, usually towards the end of winter when the trees (hi, Meyer lemon, looking at you!) tend to get cabin fever. The spray contains magnesium, sulfur, iron, manganese, and zinc and can correct a deficiency. But it is not organic, hence the guilty look on my face. 

11. Do Not Over-Fertilize Citrus Trees

Having said they need regular feeding, be aware that citrus trees can be harmed by synthetic fertilizers like crack-for-plants blue-crystal Miracle-Gro. They tend to burn plants easily if over-applied. Organic fertilizers are more forgiving (and easier on the planet, in terms of how they are produced). 

Follow the dosage instructions on the bag, for pot diameter or volume of soil. And avoid fertilizer sticks: They can burn the roots closest to the fertilizer.

Signs of over-fertilizing: 

Crisping or burning of leaf-edges. 
Slow or no growth.
General malaise and leaf-drop.


12. Citrus Trees Need Sunlight (or Very Bright Light)

When we were searching for a new apartment one of the requirements it had to meet was for a sunny room where the citrus trees could overwinter. Luckily, we found an ideal spot, and our top-floor, south-facing bedroom with lots of windows is home not only to the citrus but to galangal and bay tree. An indoor jungle.

Most citrus trees require six hours of direct sunlight a day (eight is even better but with six you will be fine).

If you must keep citrus in a less-than-bright room, boost the light with grow lights. You will need at least a couple per tree. If you have a darker situation, full spectrum lights are very helpful.

But Aaron Dillon, the owner of Four Winds Growers (where I bought my finger lime and yuzu) in California, says that in cases where a tree receives "at least some natural sunlight" during the day, "a simple T-2 fluorescent can provide enough supplemental light."

Signs of low light: 

Poor growth.
Very slow water uptake. 
Green leaves drop from the tree.


13. Too Much Light Stresses Citrus

We're not talking sunlight. In an excess of zeal, some indoor growers may give the citrus tree 12 hours or more of light, using grow lights. "This causes the plant to go into a vegetative state," says Aaron. Think of them as sleep-deprived. They are very stressed.

Eight hours of grow lights is more than sufficient. The goal is to mimic the natural season as much as possible because the citrus trees have specific seasons of flowering and fruiting. Too much light can interfere with a normal production cycle and limit its ability to produce fruit.


14. Temperature for Indoor Citrus Trees

My trees spend seven months indoors. I move the citrus flock inside when the forecast for the week ahead shows that nights dip consistently below 50'F. Most citrus are hardy to much colder temperatures and can even stand a brief freeze, but my goal is to maintain their overall, optimum health. Moving a citrus tree from very cold temperatures to a relatively warm indoor space can shock them, and may result in a big leaf-drop. No leaves means no food-factory for the plant, and also a weakened immunity to pests. 

Watch the forecast, move them once, and keep them indoors for the rest of winter. 

Indoors, it helps if you keep the nights cool. Cooler nights give the trees a sense of chilling (or a mild winter, which is still part of their natural cycle), which is helpful for flowers. (FWIW our thermostat in the day is set to 68'F, but at night it dips into the low 60's.)

Your citrus trees can also get too cold indoors. Perhaps your tree is in an uninsulated porch or garage. It may overwinter without dying, but it won't thrive, as cold prevents nutrients from being available to the plant. It's not very serious if you find that it recovers reliably once it's outside again.

Signs of being too hot and dry: 

Spider mites.
Browning leaf edges.

Signs of being too cold: 

Yellowing leaf veins can indicate that temperatures are too cold for plants to absorb nutrients.
Major leaf drop.


15. Humidity for Citrus Trees

To raise the humidity indoors I am low tech. When the outdoor humidity drops very low in mid to late winter, I mist the air around the trees - and the leaves themselves -  daily. I use an entire, small spray bottle. While a Purdue study suggests that misting is close to useless, maybe I mist better?! Or perhaps I just like misting. And, as my dad used to say: Results count. My trees seem to like it. I also find that the more plants we have indoors, the higher the natural humidity. 


To increase humidity you can also place the citrus pots on trays of gravel (or on pot feet, or a stand), and keep some water in the saucer, but never touching the base of pot. This helps raise ambient humidity without drowning the roots.

Or invest in a humidifier. My friend Kevin Espiritu (founder of Epic Gardening and author of the Field Guide to Urban Gardening) recommends a Honeywell Cool Mist humidifier “for serious plant parents” to boost humidity levels significantly.

Signs of low humidity: 

Brittle, browning leaf edges.
Leaf drop. 
Spider mites.


16. Citrus Trees Hate Drafts and Blasts

Keep your citrus trees away from hot air vents and radiators. Placing them near regular cold drafts, like beside an exterior door, is also not a good idea. Extreme changes in temperature stress citrus. Stressed plants (like people) get sick.

Signs of air stress: 

Browning leaf tips.
Leaf, blossom and fruit drop (some blossom and fruit drop is normal).
Spider mites.


17. Scale and Spider Mite on Citrus

Speaking of mites. 

Check your trees daily. A little honeydew and a few cobwebs will turn into an infestation (of scale insects and spidermites, respectively). This is the only time you have to leap into action and invest real time on the trees. The scale above was after a long winter indoors. I found it only as the trees moved outside in May. The little suckers eluded me on the  side of the tree closest to the window in a deep windowsill.


Luckily, that time, I had help. Migrating common yellow throats snapped up all the scale I had missed. Magic.

Indoors, be methodical. If you see clear sticky spots (so-called honeydew from scale) on leaves, inspect your tree all over. Ditto is you see ants rushing up and down (they are attracted to the sweet honeydew). Check along the midribs of the leaves, top and bottom. Scale may also lurk on new growth, on thinner branches, and in branch-joints. Remove scale by crushing with a finger nail (I know, but trust me, you will want to), or swab each scale insect with rubbing alcohol on a cotton bud, or scrubbing them off with an old toothbrush. I prefer squashing. It is so final.


For a serious scale attack I wash the scale off the leaves with liquid dish soap that contains a degreaser. I gently use the rough side of a Scotch Brite sponge and even a tooth brush for tricky crevasses. Just don't scrub the leaves so hard you scratch them. 


It takes me about 25 minutes to wash every single leaf on my Thai limes. Tedious, but in the big scheme of things? Not so bad.


Afterwards, spritz the tree soap-free with a spray bottle of water. Or rinse the whole tree in its pot in the shower (cover the soil with a plastic bag to prevent soap from washing in).

If scale keeps returning, spray the leaves top and bottom with diluted Neem oil and repeat every three days until the invaders have left. For me this is a last resort, but inevitable. Neem is just so damn sticky. I also do this in the shower, for easier clean-up.


If you see the tiny cobwebs on the leaves, you have spider mites. The chances are the room is too hot and dry. Prevention of spider mites is easier: Lower the ambient temperature if you can and boost the humidity. Mites love hot dry conditions. Try dish soap (with a degreaser) diluted in water to spray the mites. The only really effective longer-term treatment I have found is Neem oil, diluted and sprayed on the leaves - top and bottom - and branches.


18. Citrus Trees Do Lose their Leaves and They Can Recover

Although it has never happened to my trees, it is common for citrus trees to lose leaves when they first transition from outdoors to inside. I think the reason my citrus trees have not lost leaves is because I bring them in, as mentioned previously, when overnight temperatures dip below 50'F.  Yes, you can wait longer, as citrus are hardy down to a brief freeze. But the shock of the temperature difference between indoors and out can cause leaf drop. Pay attention to overnight temperatures in early to mid autumn.

A few slightly yellow leaves that drop off can be normal: the tree moves chlorophyll from older leaves (which turn yellow) to new growth or fruit.

If leaves do drop, don't panic and start over-watering. Only water when the soil is dry (it will take much longer to dry with no leaves to help suck it up). The tree will usually recover.


19. Repotting Citrus Trees

Your new, young, growing tree will need a larger pot every year or two, just an inch or two larger than its current diameter. 

The ideal time to repot is when the tree is dormant, but indoors they keep growing! I repot in spring after all fruit is harvested. But I have also repotted in an emergency (root rot) in early summer, and with care the trees were fine. 

The soil mixture for citrus has to drain very well, and I like to mix potting soil (never topsoil) and orchid bark, in a 50:50 blend.

To repot, lay the tree on its side, loosen the outer roots gently, and repot in a larger size, adding some fresh potting soil and bark. Water at once. And then not again until the soil has dried out.


20. Citrus in Pots Need Periodic Root Pruning

Any potted plant can become rootbound (see the picture above). The roots keep growing, but have nowhere to go, and gradually fill the pot, circling the edges. This is called girdling, and the tree's health will begin to suffer. Pruning the tree's roots so that healthy feeder roots can grow again and absorb sufficient nutrients is necessary every few years (two to three). I no longer want to keep up-sizing my citrus trees' pots (because they will become too heavy to carry), and so I root prune them regularly to manage their size and health.

I know when my Thai limes trees are due for a serious prune, top and bottom, when they seem very top-heavy and when their pots dry out much faster than in the previous year. 

To prune the roots pull the whole tree gently out of the pot (see how I did it for our fig tree), lay it on its side, trim about two inches off the rootball all around, and repot with fresh potting soil and bark. Water at once.

The branches must be pruned at the same time, to reduce shock (suddenly there are fewer roots to deliver moisture and nutrients to the leaves) and to manage the size of the tree, of course. I cut them back by one third. There is an art to it and it helps if someone experienced can guide you the first time you do it. Otherwise, always prune at an angle right above a leaf or where another branch joins the one you are removing. Use proper pruning shears (they shear, rather than squeeze, the branch). I like Felcos.

Signs that you need to repot your citrus tree:

The trees need water more often than they used to (because there are more leaves and more roots).
Leaves begin to drop.
The pot feels light for its size (more roots than soil).
Plants are top heavy and tip easily.
Roots grow right to the edge of the pot and wrap around its interior.


21. The Pot Can be Too Big

Overpotting is when a pot is too big for a small plant. The soil-to-root ratio is too high and the roots cannot absorb all the moisture in the soil. This creates damp conditions and as we now know citrus trees hate being wet. In my opinion the ideal pot size is no more than two inches wider (on all sides) than the rootball of the tree.

Signs of overpotting: 

Constantly damp soil.
Loss of leaves.
No new growth.
Failure to thrive.


22. Pay Attention

Having you seen the movie Being There, with Peter Sellars? It's about a gardener, and the title sums up the most important aspect of gardening, and perhaps life. Being there.

If you do not check on your citrus trees every day, even when they are healthy, you will not notice changes.

Think of it as forest bathing, and therapeutic. Look at your tree/s. Observe the overall form. Is the tree looking well shaped and perky, or droopy and off? Are the leaves shiny, or dull? Go closer. Yellow leaves? Crisping edges, a few cobwebs? Or just glossy and dark green?

Winter is the time to be watchful.

Signs of citrus problems are diverse. But if you don't see them, you can't address them.


Does this all sound like a lot of work? It can be. But tending your little citrus trees has tangible results and genuinely therapeutic value in an age when our attention is focused on abstract screens. Have you ever made lemon blossom bitters, above, with the dropped blossoms after the tiny fruit have formed? You could.

In all my gardening life, I don't think anything comes close to giving me the pleasure of my citrus trees. The perfume of lemon blossom for weeks in the middle of winter, and the reward of waiting for fruit to ripen in its own time, are exquisite.

____________

No comments:

Post a Comment

Comments on posts older than 48 hours are moderated (for spam control) . Yours will be seen! Unless you are a troll. Serial trollers are banned.