Growing Citrus Indoors

Growing citrus indoors may not be as hard as you think.  However,  there are unique environmental factors to consider. There is a different type of intuition one must develop for citrus to not only survive but thrive indoors. This is accomplished, as with so many things, through practice.

The move indoors is the single most important event to plan for every year. This big change can have a number of cascading side effects. They must be dealt with promptly and completely. First and foremost we want our trees to survive if nothing else. Ideally, we want to reduce fruit drop, leaf drop, and bloom drop too. Depending on how suitable your environment is for citrus, we could force blooming and produce fruit out all while indoors. This should not be your expectation the first go around, however. Once we feel like things are stable or perhaps in an effort to create a stable ecosystem, we can introduce automation systems.

Allocate substantial time during the weeks following the transition inside. To reiterate, this is the one single event you can bank on each year where things will go wrong. The extent to which things go wrong is within your power to control. Our goal is to mitigate the inherent negative impact and have as happy of trees ensuing.

Measure your Space

How much available floor space do you have in your home? If you are dealing with a modest number of square feet, you’ve probably already been creative in arranging your other possessions to fit the space.  The placement of your collection in your home, while perhaps challenging, will probably be a familiar exercise.

Take out your measuring tape and record the width and length of your trays if you don’t already know. Factor in how much the tree’s canopy goes beyond the radius of the tray. We don’t want trees so crammed so that they’ll grow into one another or block light.

If your ceiling isn’t that high, measure the height of your tree from the floor. A taller, more established tree may need to be pruned to fit through the doorway. The doorway is usually the reason why any additional pruning would be needed in the fall.

Which direction is your house or apartment oriented? How many windows are south-facing? How big are those windows? Is aesthetic important to you?

Are you using a self-watering system or some other automation system? Does it require your plants to be in close proximity to a set of microcontrollers or other equipment? Does the design of your self-watering system require all trees to be in a circular configuration around it or at an end of a row or rows of trees?

Controlling Temperature

The prerequisite to controlling temperature is knowing the temperature. You can buy a temperature monitor which many are very affordable. These units will also include relative humidity which is another important factor in having happy trees. The affordable ones will even list out the min and max for both the temperature and relative humidity over the last 24 hours.

I oftentimes read that leaf drop can be caused too low of temperatures. While this can be true

Aeration

Aeration indoors during the winter does not compare to natural airflow your trees experienced during the summer. To compensate, we can use fans to create an artificial window to simulate the outdoors. Wind also facilitates in distributing moisture so that your trees are surrounded with adequate humidity.

Finding the Right Fan or Fans

Not all fans are created equal. A little Honeywell circular fan may be useful in making you feel cool in a home without central air, but that’s not going to pass mustard for adequate airflow for all but one maybe two trees.  Generally, a larger high powered fan is a better fit. With over a couple dozen trees, multiple fans are required.

I use four fans split between two rooms. Two are barn fans and the other two are high-velocity fans. Size of fan does not necessarily mean increased air circulation. In fact, my barns fans are a couple inches shorter than high-velocity ones but are much more powerful.  As I like to say, if it’s good enough to cool a horse, it’s good enough to move air for my trees.

Why is having a breeze important?

Air circulation is key for your plants to thrive. Photosynthesis requires carbon dioxide.  Airflow facilitates carbon dioxide to be readily available for your plants. Circulation is also key in moving moisture throughout your house.

Programming Fan Cycles

Each of my fans is controlled by a smart plug. This gives me optimal flexibility and saves me some on my electricity bill. During the daytime I have each of the fans turn on for 20 minutes and off for 15 minutes circularly.  At night, I have the fans turn on for 3 minutes and off for 50 minutes. This nighttime schedule is mostly too keep the noise minimal during sleeping hours but still allow moisture to move around the room at night (I always have my humidifier run at night).

Regulating Humidity

Try keeping indoor relative humidity in the range of 40% to 50%. During winter, when we heat our houses, the loss of humidity is tremendous. Depending on your house’s heating system, levels of humidity to vary. A radiant water heating system will have a relatively higher humidity than a house warmed via ceramic space heaters for instance.

You can increase the humidity for your plants in three ways: using room humidifiers, spraying water on the citrus leaves, and placing rocks in-between your tray and the pot and filling the tray with water.

Many residents in Minnesota use a humidifier in their home to keep their skin from drying. These humidifiers are an especially useful tool for regulating the humidity around where you citrus trees are situated.

Programming Humidity in the Home

Fulfilling Sunlight Requirements

Indoor sunlight requirements must be satisfied by either natural or artificial sunlight or both.  For those growing in northern climates, artificial light is needed under most circumstances. In Minnesota, grow lights are a necessity to make it through the winter. The worst condition your trees will be during the year is right before you put them outside in the Spring.

There is nothing better than natural sunlight. The best, most technologically advanced lighting system can’t beat the sun and it doesn’t seem like that will change anytime soon. Despite this, we can successfully provide our trees with the right parts of the light spectrum for the right duration during the right part of the year. No doubt overwintering trees can be stressful. Fulfilling sunlight requirements is one of the most fundamental issues with growing citrus indoors.

With so many trees and relatively few windows I need to supplement with grow lights. The shortest days of the year will be around the winter solstice. In Minneapolis, Minnesota, the sunrise is 7:30am – 8:00am between sunset is sometime between 4:00pm – 4:30pm. This means the shortest day is roughly 8.5 hours. Keep in mind this not direct sunlight, but potential sunlight that trees can use.

The duration of day (daylight) is different from the hours of sunlight. Sunlight hours are impacted by clouds. We can take an average of sunny/cloudy for any given day in December(which turns out to be in the low 40s) and multiple that percent against the total daylight hours. On the winter solstice, this ends up being about 3 and 3/4 hours of sunlight.

Now we need to consider how much sunlight actually makes its way through your home’s windows and onto to the trees. Depending on the orientation of your house, the number of windows, and size of windows, we could be talking about a lot less sunlight.

citrus sunrise winter solstice

citrus sunset winter solstice

While sunburn could happen outdoors in Minnesota, it’s probably more likely that you’ll sunburn will occur when you place a grow light too close to the tree. Have a couple feet of spacing from your grow lights.

Citrus Tree Arrangement

Some citrus trees do better when cross-pollinated with other species. For example, Minneola practically must be cross-pollinated with something like a Sunburst Tangerine. Others like the Clementine perform worse. Do your homework.

In the winter time, most of my trees will not be in the bloom cycle, so this may not even be a consideration. If your citrus trees do bloom indoors, you may need to pollinate them yourself with a cotton swab. While outdoors the wind and bees do all the work, but unless you have a bee hive or some barn fans indoors, don’t count on any fruiting without intervening.

One important factor to considers is position. Drafts that come from windows and the perimeter of your house will cool the area near them. Try keeping your more cold tolerant citrus close to the outside walls in this order kumquats, satsumas, mandarins, oranges, limes, and then lemons.

Timing Your Move Indoors

Use the Boy Scouts Motto: Be Prepared.

It’s debatable as to what criteria should be used as to when move your trees indoors. This is a bit of an experiment for me at this point. In two to three years, I’ll have some empirical data to back up the process. Based on 2018, I wouldn’t move your trees indoors any earlier than mid-September. There are simply to many swings in temperatures earlier than this. In 2019, I plan on delaying as much as possible and probably move them in at the end of the month.

Some things to consider:

  • Citrus grow best when temperatures are 75-90 degrees Fahrenheit.
  • Leaf drop primarily happens due to rapid temperatures, a significant difference in humidity, or decrease in sunlight.
  • Excessive leaf drop means your tree is stressed and unhappy
  • A house stores heat. Your home’s temperature is higher indoors than outdoors. The longer you wait in the year, the higher the difference between indoor and outdoor temperatures. Before temperatures reach 50°F at night, think about bringing them indoors. Look at your indoor thermometer and subtract it from the outdoor temperature difference. Every house is different, but try to keep that number in the range of 10-15°F degrees for optimal conditions.
  • The nighttime is cooler than the daytime

Based on the above points, the time of day for my move is going to be the evening.

Watch the weather forecast closely. During late August in Minneapolis there’ll be a couple high 80s or low 90s days. I figure that these are some of the last best days of the year for my citrus outdoors. Following these days when the temperatures go back down into the low 80s for a high, I’m going to move them indoors. Once outdoor temperatures dip into the 70s during the day, my now housed trees will still be in the 80s (during daylight hours), essentially extending the optimal temperature range for longer. At least this is my theoretical plan.

Move them all in at once or in batches?

If done in batches over multiple weeks, there probably should be a pecking order.  Kumquats are the most hardy, so I’d leave those at the end of the list. What about trees that already have fruit or blooms vs ones that don’t.

Todos

If using trays or saucers outdoors, you’ll want to clean them before using them indoors.

trays laid out in backyard to be cleaned in preparation for the fall move in
Trays laid out in backyard to be cleaned in preparation for the fall move in

 

Reorganize your space and place the trays down before you actually move the trees in. Consider placing down a towel under the trays to prevent scratching if on hardwood floors.

Use a hand truck or dolly to move your trees from your yard to an entrance of your home. Then move the hand truck or dolly indoors and use to move them to their trays. This may seem unnecessary if you have a couple of trees, but with more trees, you’ll save your back.

After you are satisfied with the placement of your citrus trees in your home, reposition your grow lights so that all your trees are getting light. Next, plan out a schedule for when to turn and off your lights. I try to simulate the timing of my grow lights based on the time of day and which side of the way they are facing. My main citrus growing room has 2 grow lights. The window is east facing. This means that my plants will receive the most natural sunlight in the mornings. Since this window is not southern-facing, I supplement it with the artificial light right away in the morning in sync with the sunrise. This grow light is against the western wall of the room. In this way, light is reaching my plants from both directions.

When morning transitions into afternoon, most natural sunlight is lost on the eastern window.  This is when I turn on my eastern grow light in the main room. Now, artificial light is coming from both sides.

In my smaller room, the window is southern-facing. So it will receive natural sunlight throughout the day. The concentration of light will be most in the western side of the room and transition to the eastern side of the room since the sun moves from east to west. In the later afternoon, I supplement with a grow light and run it for several hours each day.

Experiments

On August 28, 2018, I moved five citrus trees indoors. The weather conditions prior were cool, reaching down to mid-fifties outside at night, and overcast for days with some rainfall. The five trees were my Key Lime, Brown Select Satsuma #1, Washington Navel Orange, Owari Satsuma, and Valencia Orange trees. I selected these trees because they all have multiple shoots of new growth coming off them.

Since my house should theoretically be warmer than outside in the next week or so, I think these trees will actually be better off indoors at this time of year. The temperature inside my home should be more conducive to growth. I am running a fan next to them on the low setting to produce a nice breeze. I also have an evaporative humidifier in the space that’ll I’ll turn on once humidity levels start declining. I have three large windows oriented towards the east for natural sunlight.

Simultaneously, I’m supplementing with a full spectrum of fluorescent lights on the opposite western side of the room. Despite being indoors, this additional lighting coupled with the natural sunlight received outdoors in now clear skies should offer more photosynthesis than immediately before these trees’ move indoors. I am going to be judicious about monitoring leaf drop. The indoor temperature is pretty much right in line with current outdoor temperatures.

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