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This post contains excerpts from my book, The Complete Mini-Guide to Growing Windowsill Lemon Trees: A Reference Manual for Northern-Zone Gardeners. The book is a comprehensive instruction manual for growing potted lemon trees indoors, and may be purchased on Amazon in ebook or paperback formats for less than the price of a fancy cup of coffee!

Growing lemons is one of my favorite gardening hobbies, and I’ll be honest: that’s mostly because it sounds so impressive and difficult, but in reality it’s practically hands-off. While all my gardening buddies look up to me for growing and actually harvesting from these subtropical citrus trees in a zone where temperatures regularly fall below zero, the cold hard facts are that I just set the tree on the windowsill every fall and practically forget about it until spring.

However, there are some strings attached. Before I could get away with this low-maintenance lemon tree, I had to create the right environment for it to thrive. Creating the perfect windowsill environment to grow a lemon tree requires you to tackle a host of survival elements, but in this post I’m going to focus on two of the most important: temperature and humidity.

SELECTING THE RIGHT TEMPERATURE FOR WINDOWSILL LEMON TREES

Depending on who you ask – who knew that growing citrus was so controversial? – a lemon tree will be more cold-tolerant than the lime, but less cold-hardy than the typical orange tree. A lemon tree’s target temperature range is roughly in the 70s-80s range, and if given reasonably supportive conditions it is typically pretty tolerant of temperatures that climb well into the 90s or low 100s.

However, don’t make the mistake of thinking that your tree is pining away for subtropical temperatures all year round.

In winter, a windowsill temperature of 65 degrees is perfect acceptable and, in many cases, considered ideal. That said, when summer rolls around, give the tree a chance to experience as natural an environment as possible; in many northern locations, hanging around outside during at least a few of the warmer months will do the tree good, even if you aren’t expecting temperatures of 95 all summer.

Outdoors, lemon trees are tougher than you might think. If only exposed for short periods, the trees can often tolerate a very brief, very light freeze (and by “tolerate,” I mean the tree itself will probably survive. You can expect the lemons themselves to be ruined at any temperature below freezing).

Don’t push this boundary – don’t even get close to it! – but if caught off guard by an unseasonal frost, you might not need to panic just yet. To be safe, be prepared to bring the tree in for the winter season when the temperatures start dipping below 50 degrees Fahrenheit. 

NOTE: The younger a tree is, the more delicate it will be. Just as you wouldn’t expect an infant to have the same cold tolerance as an adult, you can’t expect a baby tree’s fragile, flexible leaves and stems to survive a frosty night even though a mature tree with tough woody stems might be able to sail through the night with minimal damage. The younger the tree, the fewer dangers you should risk exposing it to.

One note of interest – you can use temperature fluctuations as a way to induce blossoming. In its natural environment, the tree responds to certain environmental triggers that prompt it to flower and set fruit. Lower temperatures at night are an important catalyst, so if you’re looking for fruit, aim for cooler nights (figure ten degrees Fahrenheit below the daytime temperature, although five degrees might be enough to do the job).

Not sure how to ensure your blossoms produce fruit? Hand-pollinating container-grown lemon trees may be necessary. 

A key factor in cold survival is dormancy. If your tree is sound asleep, a cold snap will have less impact. If, however, your tree has been fooled into believing spring has arrived – maybe you watered those sleeping roots with warm water? maybe a move into direct sunlight for too many hours has mimicked a spring lengthening of the days? – a cold snap can be more serious.

Be thoughtful in your actions; once you’ve severed the tree from any connection to its natural environment, you have the responsibility to control all artificial environmental signals. The tree will work with what it’s got to determine when to sleep, wake, blossom, set fruit, or push out new leaves… and when on your windowsill, all it has got is you. No pressure!

ESTABLISHING THE RIGHT HUMIDITY FOR WINDOWSILL LEMON TREES

The lemon tree is an interesting study in balance. A common problem with indoor growing is a humidity level too low for optimal tree health. However, high humidity causes or contributes to a host of problems ranging from disease – which spreads more easily in humid environments – to abnormal fruit color and peel texture.

This post concentrates on low humidity, which is the more common hurdle for northern-zone gardeners trying to grow lemons on their windowsill.

Don’t confuse air humidity with soil watering needs. Choosing the correct pot size & watering schedule is critically important for container-grown lemon trees.

Colder climates naturally tend to bring down humidity. Although semi-dormancy provides you with more flexibility and requires less of a greenhouse-quality to the air, your windowsill’s humidity will almost certainly be lower than a lemon tree’s natural environment. While a humidity just a touch lower than normal is something your tree can handle, extremely dry air can put unnecessary stress on the tree.

If you see leaves start to curl up at the edges, that’s a sign of dryness in the soil or air; the tree is trying to conserve moisture. Lower humidity also provides an excellent environment for insect infestations, particularly spider mites, so it’s in your best interests to monitor and address the environment before it starts giving you other problems.

To increase humidity, start with location. Don’t place your tree on a windowsill in a room with a wood-burning stove, if you can avoid it. Find a room away from dry heat sources without sacrificing lighting. You can build a mini-greenhouse with a plastic sheet, but I’ve never found it neccesary to take this step; unless your air is exceptionally dry, a mini-greenhouse can potentially cause problems for a tree that’s trying to ride out the winter in semi-dormancy.

The simplest way to raise humidity is to invest a dollar or so into a spray bottle, fill it with plain water, and spritz some around the foliage on an as-needed basis. Don’t soak the tree; just a little spray in the air and across the leaves, enough to get the air slightly damp and let any residue on the leaves evaporate throughout the day to keep the humidity semi-stable.

Before you take this step, however, make sure you are watering the soil correctly. Moisture to the roots will always have more impact on a tree than moisture to the leaves will, and all the misting in the world won’t keep a tree from dying of drought.

It may sound complicated to get temperature and humidity just right to successfully grow lemons in the far north, but I promise you, it’s so easy once you have cracked the code. Figuring out the correct environment for these subtropical fruit trees is the hard part; achieving that environment is easy! (For more information about the lemon tree’s ideal growing environment, check out this FAQ article on how to grow potted lemon trees indoors.)

There is a LOT more to growing windowsill lemon trees than just getting the temperature and humidity right… but without these two elements of success, your citrus gardening efforts aren’t going to go anywhere. Fortunately, the lemon tree is a very adaptable and forgiving tree, and as long as you put in a real effort to learn what it does and doesn’t like, it will reward you with longevity and lots of zesty fruit.

Interested in learning more about growing potted lemon trees indoors? Check out my book The Complete Mini-Guide to Growing Windowsill Lemon Trees: A Reference Manual for Northern-Zone Gardeners (available on Amazon along with its companion book The Complete Mini-Guide to Growing Windowsill Pomegranate Trees!) or read more excerpts here:

This post was originally published in 2022. The post has since been updated to keep information and links current.

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