Citrus trees need 6-10 hours of sunlight for the entirety of their lifetimes. This might be the least negotiable of all the requirements to grow citrus; i.e. it will not survive without enough sunlight, regardless of whether you water it perfectly in a warm environment in fertile soil.
Direct sunlight must be available. Here’s the twist, though: because it’s possible for them to get sunburned or heat-stressed, ideally there’s an option for bright indirect light in case it ever needs to cool off temporarily. If it’s showing signs of heat stress, move it to indirect sunlight for a day or more, and give it a full watering if needed. The reason they’re tolerant to some indirect light is because citrus trees originated and evolved as an understory species starting around 8 million years ago in the Himalayas in southern China. I discuss the genetics and some of the evolutionary history of citrus species in another post.
South facing windows with access to consistent daylight from east and west are ideal. I happen to have this exact setup in my living room, and in Zone 7b it’s just enough. In peak conditions in the spring, summer, and early fall, it needs 8-10 hours of direct sunlight. In the late fall and winter months, strive for 6-8 hours of direct sunlight. When natural sunlight isn’t available (e.g. in zones 7 and below) an artificial grow light is recommended to make up for the loss.
Citrus trees thrive between 60-90F. Mild to moderate humidity is ideal, and you can substitute with misting or use a humidifier in a dry climate (or, a nyc apartment microclimate). Generally, the warmer the better as far as temperature goes. Choose the least-drafty, most consistently warm and sunny spot you have available. However, putting it directly on a radiator is not ideal and is likely to kill it off.
I do not recommend keeping it in an excessively damp environment (like inside a shower), as the roots prefer to be dry and are highly prone to rot when too damp. And while I just said heat is better than cold, excessive direct heat isn’t ideal either, as the shoots will experience sunburn, heat stress, and wilt at temps above 110F.
Citrus trees thrive between US Zones 9, 10, and 11. In the US, these are the only Zones where they can be planted and grown outdoors year round. Below are USDA hardiness zone resources, but the NY tristate area is in Zone 7. Here, growing citrus trees will be a largely indoor endeavor.
Here’s a cheat sheet of the USDA growing zones in the northeast:
The NYC area and the tristate area (including NJ and the Philadelphia, PA region) is in Zone 7b.
Just north, the area from Hudson Valley up towards Albany, as well as the neighboring region of southwestern New England (western Connecticut and Massachusetts), is in Zone 6a.
Farther north, towards upstate NY (i.e. the longitude between Saratoga to Syracuse and Rochester) is designated as Zone 5b.
Eastern Massachusetts in the Boston metro area and back bay, Suffolk County, Plymouth County, and Cape Cod are between Zones 7a to 7b.
Look up your growing zone on the USDA website: https://planthardiness.ars.usda.gov/
This article from NPR also has useful information about the recent updates to the zone map: https://apps.npr.org/plant-hardiness-garden-map/
In short, do not plant this tree outdoors in Zones 5-7 and expect it to survive. The winters here are too cold, too dark, and last too long. There are ways to attempt this in cooler zones (e.g. Zone 8), but it takes a lot of work, expertise, and investment to protect the trees from frost.
Do not let the young shoots sit outside in any conditions, for any reason. Even light breezes will blow over and break immature shoots, which I learned the hard way with my very first group of lemon tree shoots (RIP). When they’re very young and delicate, I don’t even recommend keeping them near an open window; the breeze might be too strong.
My very first group of lemon shoots, which I started in late March, died a sudden, painful death in late May. One day, I put them outside on my fire escape for a few hours for the daylight, and the summer breeze broke the delicate shoots and killed them all.
So, as tempting as it is to let them photosynthesize outdoors, keep young specimens indoors away from the elements for as long as possible. As they have matured 1-2 years old now, I still keep them indoors but I let them be near open windows. In the future it might make sense to move them outdoors on a part-time basis (e.g. to a sunny porch on nice days only in the summer), staking it so it’s more secure, but I don’t think they’ll be ready for that for at least another year or more.
Between July 2 and August 4, I planted the germinated seeds in basic 3.5 oz. plastic cups that I hand-cut drainage holes into, and sprouted them in makeshift mini greenhouses with plastic dollar store bins. By late winter/early spring 2025, it will be time to transplant to a slightly larger cup.
You will have far fewer to care for than I did, but here are photos of my setup for Group 1 shoots in my bathroom (about 56x). It got a little crazy in there.
