The Joy (and Pain) of Growing Citrus Indoors

It’s citrus season here in New England! At least, in my greenhouse where at least 10 varieties of citrus can be picked on any day in February. While easy to grow in a cool greenhouse, indoors, citrus need a little extra special care – here are my tips.

I’m often asked about how to grow citrus indoors or how to grow citrus in containers – and while I am hardly an expert on raising citrus fruit, I have been growing many citruses in pots since, well, when I was a kid. Really.  Here’s a secret – my very first houseplant grew from a sunflower seed one of my mom’s flower pots, but my second was a grapefruit tree that I germinated and kept growing into a thorny beast which my parents gave away (or threw into the compost pile) when I left for college.

If there is a single reason for growing any citrus indoors, it should be for the fragrance of the blossoms.

I think citrus are great plants for many to try indoors, but they aren’t foolproof, nor even ‘easy’ as many will claim. Young plants are easy to obtain, even from seeds that you find in a grapefruit, which is a good project for children to learn. I myself remember how excited I was when I germinated my first grapefruit plant, but I later learned the realities of getting citrus to bloom and fruit indoors and how it requires a grafted plant of a named variety. No need to ruin the dream for the kids though, they already are having a blast with their science project and who knows – you may have a budding botanist in your future!

Meyer lemons thrive in cool greenhouses and cool rooms indoors, ideally, they should bloom in June and the fruit will mature in winter – just when you would want them!

Australian finger limes are very fruitful, but also very thorny so be prepared, especially if you have kids!

A few facts up-front though, especially if you want to have fruiting citrus indoors. We should work through all of the facts and misinformation out there about citrus indoors before you undertake your own dreams of winter lemons and oranges. First, sure you can plant seeds of most any citrus (I just said that I did, right? I encourage it – but know that this is just a fun science project and that your hard work and years of dedication will most likely result in a thorny shrubby plant that won’t bloom for a decade or more, and when it does, the fruit will most likely be inedible.

Some citrus I picked this past weekend in our Massachusetts greenhouse. )Left to right) Ponderosa lemon, a Mandarin orange, ‘Improved Meyer Lemon’, ‘Pink Variegated Lemon’, a Limequat, a ‘Fukushu’. kumquat, a Sweet Kumquat, or ‘Meiwa’ kumquat, a variegated Calamondin and last, the tiny Hong Kong Kumquat.

I think before starting a citrus farm indoors, you should decide what you want. Do you want flowers? Decorative fruit to remain on the tree all winter? Or do you want to actually pick fruit and use it? Sure, one might say that they want it all, and yes – you will get blossoms anyway, but not all citrus is the same. Let’s go through the list of popular citrus that you can buy and see what is most growable in the home, or in a greenhouse.

‘Mandarin Orange’ which includes those sold under the name of ‘Cuties’, ‘Clementines’ or any of the classic varieties like Tangelo – these oranges are all related, have east-to-peel reticulated skin and are delicious, but they are more challenging for culture indoors. Forget about starting seeds from those clementines and getting oranges in the house on your windowsill. It’s fake news.

A nice mandarin orange freshly picked from our tree last year.

There are named varieties of some Mandarin types, which you should seek out – Logee’s Greenhouses in Connecticut carries some but between you and me, unless you have a cool room to grow them in, they are better for cold greenhouses. These varieties produce delicious fruit, but let’s be realistic – the trees grow larger than most citrus, and are more difficult to grow well indoors, at least in the typical North American home. If you want to try, you must grow them in very bright light in the winter with the coolest temperatures and moist air. An unheated mudroom – one that stays above 35° F but below 55° F is ideal, or a garage, breezeway, glassed-in porch that does not freeze – anything with a slate or concrete floor – you get the picture. One could say this about all citrus, really. A house circa 1850 with radiators would be ideal. No wonder those Victorians had so much luck with camellias and lemons.

Social media just loves ‘easy and fun’ hacks and DIY projects that oversimplify and over-promise. While well-meaning, must of the advice found is unrealistic and can be disappointing.  When it comes to growing citrus, never start one from seed if you want to be able to pick fruit. Start instead with a grafted citrus for many reasons, but mostly because of time and overall size. Citrus from seed may bloom, but it can take many years if not decades. Citrus from seed however is still a great children’s project and there is little harm in letting them dream about picking oranges!

MYTHS ABOUT STARTING CITRUS FROM SEED

There is this image that’s been going around on mommy blogs and even some DIY gardening blogs that shows a cluster of lemon seedlings growing in a teacup along with the headline’  Grow your own room freshener!’ I’m not usually a grumpy dude but that image drives me crazy because it is such bad advice.  Can you germinate a handful of citrus in a teacup? Sure. But the only way that it’s going to act as a room freshener is if you totally smash the leaves until they are crushed. Is it long-lived? Of course not because crushing the foliage will kill every seedling. Don’t get me started. For some reason, no one seems to call out these fake bits of advice, but I thought that if you don’t know already, that maybe you would like to know the truth.

Citrus seeds as a kids science project, however, a great thing. It’s how I actually got the gardening bug – a grapefruit seed that was already sprouting in a grapefruit was planted in a larger pot of one of my mom’s houseplants, and I was so excited when it germinated (I was in first grade) that I kept that plant growing until I left for college – when my parents threw the thorny beast into the compost pile. But just know that you’ll want a good, grafted citrus (the rootstock is usually a Fortunella species, or something more hardy vigorous and allows a blooming branch – a clone, really – of a named variety that is proved to be delicious and fruitful, to bloom and set fruit as soon as the following year. Not to mention that the rootstock for potted citrus are specially selected to keep the grafted plant smaller, and room-sized.

I should mention that sometimes a grafted citrus will send out a sucker, or a branch from below the graft. Always keep a look out for these, they will be more thorny and usually more vigorous than growth on the top. Remove them as soon as you see them.

Calamondin oranges make a good option for indoor citrus. The variegated variety is very lovely both as a potted plant and as a decorative one that can be set outdoors for the summer where it will bloom and bear fruit the following winter.

Another good indoor citrus is the ‘Calamondin Orange’, popular with Philapino folk, it has been a popular houseplant since the house plant craze of the 1970’s. Look for the variegated one as it is the prettiest. The small fruit is edible, and not a bad substitute for lemon when added to tea. Like any fresh citrus, the oils in the skin alone make for a tasteful experience.

I want to mention Kaffir lime leaf, which is a citrus as well – a very handy plant to keep in the house, and outside in the summer especially if you are an adventurous cook. One leaf added to cocoanut rice or Southeast Asian dishes and curries is transformative, and it makes the often unwieldy plant worth its real estate. Just allow it to grow large first before pulling leaves because if you are like me, all the leaves are used up before the end of summer! With one or two growth spurts a year, I really need a big tree of this one!

In our central Massachusetts home which (is just over 100 years old and poorly insulated), it kind-of friendly to citrus as long as they are kept in the cooler rooms. We even have this huge unheated room and with big windows, the ceiling is 16 feet tall and there is a concrete floor – exactly what so many potted citruses appreciate ( and camellias).  Not everyone has that perfectly cold room, but some folks have a cellar with big windows or a glassed-in porch or a garage with big windows. Even an unheated bedroom will do.

Many hybrid kumquats are now available, a(this one is Fortunella obovata ‘Fukushu’, or the Changshou Kumquat. All kumquats make delicious and fruitful indoor citrus as long as you can provide the cool conditions they desire. In our cool greenhouse we have enough kumquats to eat fresh all winter long. This one is one of the best (it looks like an orange) but it’s round fruit are sweet, and yes – you always eat the peel on kumquats!

Kumquats became our favorite though. Also maturing in the winter, the fruit is absolutely delicious when picked right off the tree and popped into your mouth. Nothing at all like store-bought kumquats – in fact, I would peel kumquats if I ever had to eat one, and I would never ever know what a treat they were until I ate one whole right off of the tree.

The citrus also never seem to get insect problems in the greenhouse. Scale, mealy bug and particularly spider mite problems seemed to plague us every winter no matter what we do. In the cold, damp and sunny greenhouse, the foliage remains dark green and healthy.

Citrons come in many shapes and sizes. More of a novelty than anything else, a football sized ‘Etrog’ can be an impressive show stopper. Beware though, these plants grow large (10 feet tall in a couple of years) and the thorns are deadly – I mean – poke your eye out deadly. I always have to set them in safe areas with no traffic from us or the dogs. Still, every year someone gets an arm scraped or a scalp scrath that requires stitches!

IS a greenhouse essential? Of course not, and I’m not trying to make you feel bad about not owning a greenhouse at all (believe me, you wont want to pay the heating bills!), but I wanted you to know what I have experienced when it comes to citrus culture, and it changed dramatically once I moved plants from the house into the greenhouse, and there are some learnings that came from that. Indoors, citrus certainly can be grown, but now I try to replicate what they experience in a northern winter greenhouse as best I can. I think I always felt that citrus were southern plants, trees that liked heat and summertime temperatures, but really, they like a cool, Mediterranean climate, wet in winter and cool, but hot and sunny in the summer. I mean – no wonder they love California!

Last weekends’ pickings included 9 types of citrus from the greenhouse.

Today we grow about ten citrus varieties in the greenhouse at any one time, and while everything isn’t perfect (I lost 2 kumquat varieties this year and one lemon tree due to the hose being too far away and the heater burning one to a crisp), but each reason was due to operator error. I also house a few citruses from friends over the winter – a sort-of boarding school for citrus, but I always have to be careful as more often than not, they come covered with spider mites and mealybugs. The cold temps in there keep those pests at bay but I have to isolate the plants and scrub the stems and foliage.

Would I grow citrus indoors again? Maybe, especially since the greenhouse is really getting too costly to heat (this may be the last winter I splurge on it), so plants could be relocated to a large unheated room that we have (the studio) where I think they would at least survive the winter just fine.

Fortunella hindsii, or the Hong Kong Kumquat has a near cult following by collectors, but not to eat, as the fruit contains just one seed or two, but is mostly skin and no larger than a large pea.

My favorite citrus happens to be any or all of the Kumquat varieties (‘Fortunella species, and for named varieties, there are many), as well as the tiny, inedible Hong Kong Kumquat (Fortunella hindsii) which we grow purely for novelty sake. Who could ever resist its 1/4 inch ‘dollhouse oranges’ but be careful of its thorns – this wild species is thornier than a crown of thorns!  A large topiary of this plant sits on a high bench in the greenhouse and I have to warn visitors to duck, or their head can become scratched. If it’s hard to find, try asking for it at a bonsai nursery but then pot the rooted cutting up as a tree in a large pot and don’t tell them! It’s a popular bonsai specimen.

Some of the most aggressive growers I’ve had include the pink variegated lemon (not very edible but the foliage is pretty) and the Australian Finger Lime, which quickly grew in the greenhouse into a large and thorny shrub, eventually having to be tossed as no one wanted to move it anymore. I did buy a rather expensive Mandarin orange that grows very edible and large mandarins, now trained as a sort of standard in a large tub, it’s one of those citruses where I hate picking any fruit just because it looks so nice on the plant, but I’ve learned that picking fruit on all citrus is important especial;y if you want them to bloom again and on time.

POTTING-UP A CITRUS
Many sites also advise one not to overpot citrus, and that they enjoy being potbound. I will say that we pot-up citrus the first few years into larger pots, especially if they are Logee’s plants which often come in  2 to 4-inch pots. They are still young grafted plants and rather fierce growers but are watered every day or even twice a day at the nursery because they are pot bound. While citrus will always fill a pot with roots, most of these roots are at the surface, much like a camellia in a pot. Once settled into a 14-24 inch pot a grafted citrus will remain in that pot for much of its lifetime, but any citrus you buy in a  2- 4-inch pot will need an upgrade to a larger pot almost immediately.  I would move small citrus into an 8 or 10-inch pot as soon as I get home from the nursery, and then once that pot is pot bound, move it to the14 or 24-inch pot where it will stay for at least 5-10 years, with biannual refreshing the outer soil.

Citrus has surface roots which will eventually take over the top of the pot, making a pot look like it’s pot bound when it actually is not. I topdress most citrus once a year with a bit of new soil (always using Promix BX or PRomix HP (High Porosity) which I think these acid-loving plants appreciate. The plants are fertilized only in the summer with a balanced chemical feed (RapidGro) as they like high nitrogen during foliar growth, and then hit once or twice with a commercial citrus feed. Every year we get yellow leaves because of Iron deficiency or boron deficiency, which is when we repot a plant usually into the same pot, but with fresh soil.

FEEDING, SOIL AND FERTILITY
Fertility with citrus is tricky, especially with container grown plants as most citrus fertilizer is sold for use outdoors, on trees planted directly into the soil. I prefer a water-soluble or time release feed (or both) on potted citrus, but soil chemistry factors are as well. All peat or coir based potting mixes will change physically over time as they decay, affecting pH and soil structure so they often need refreshing. Many recommend a low analysis like 5-2-6 which is similar to the Espoma citrus food but be careful if using this in a pot as it can burn roots near the surface. If using, sprinkle the granules on in the summer on top of fresh soil that you’ve added to the pot. Miracle-Gro for acid-loving plants works well too but look for high nitrogen and high potassium.

CONTROLLING INSECTS
As for troubles with insects, unfortunately, scale and spider mites are difficult to control without insecticide, so if you plan to eat your fruit of have issues with using a systemic insecticide, the only option is to toss your plant and get a new one. Mealybug, which difficult to treat without chemicals can be scrubbed off carefully, and then the soil replaced as best you can and hope for the best. I have never, ever had any luck with any organic insecticides with these pests on citrus, but as I shared with you earlier, our plants in the cold greenhouse never seem to get any pests which tells me that they are healthier if grown in damp air at low temperatures.

HOW I GROW MY CITRUS IN A GREENHOUSE
A note about the cold greenhouse here – I keep our greenhouse set to 38 – 40 degrees F in the winter, which is how cold it gets on the coldest nights – even if it is -10° F outside. Sure, it may get colder near the floor but on a sunny day even in January, the air temperature can reach 65 or 70° F. Most of the mature citrus are in large 24-inch pots which are set high in the greenhouse where it is a bit warmer  (this bench is above my head). This provides a bit more warmth and even bright sunlight as the fruit can ripen near the glass.

I still lose a  citrus plant or two each year in the greenhouse but not because they are difficult to maintain, it’s due to my neglecting them. It’s easy to forget to water a lemon that sits on the opposite side of a walk or move a plant to the wrong place.  Even though we’ve experienced a couple of freezes (when we’ve run out of fuel) this has done little harm to the trees. One night last January the furnace ran out of fuel and the air temperature dropped down to 20° F for a few hours, but there was no visible damage to the plants. Any lower and the roots might have been killed, but what did a few plants in was that once the furnace came on, hot, dry air blowing from it killed two large Kumquat plants because  I had temporarily moved them to a new part of the greenhouse so that they would be further away from the wall – but they then sat right in the path of the furnace fan.