Tropical and subtropical bonsai are fine being indoors through winter, where they feel at home in normal temperatures and can even grow year-round at 68°F or above. For you lucky folks, just keep talking and cozying up to your plants this winter.
Outdoor bonsai however need outdoor temperatures in winter in order to rest up.
CHILL FACTOR: Temperate climate woody plants need consistent temperatures below 50°F for 4-6 weeks. Ideally, this means an unheated garage or room of the house, where they get cool temperatures without the harsh winds or strong sun. Keep your tree in the goldilocks zone between 20°F and 50°F during this time. Why? Above 50°F for several days could bring your tree out of her deep-sleep. And if you're like me, expect a cranky response when that happens. Out-of-dormancy growth will stress out and eventually kill your plant. All of our outdoor bonsai are reliably hardy down to 15°F, but below that, root damage can occur. Roots have a natural anti-freeze they develop by storing up sugars in the weeks leading up to winter, but in very cold weather, even they could start to freeze. What to do? If air temps are expected to fall below 20°F for several days, add extra protection to the planter by putting it in a cardboard box with newspaper stuffed around the sides and top. Or, bring the tree to a more protected location (that still stays below 50°F).
LIGHT and WATER: Deciduous trees, like maples and elms, lose their leaves, so they don't need light (or that much water). Needled evergreens need very little light when the temperatures are below 32°F, as again, they're basically just taking it easy until spring. And those sexy slender waxy needles also help reduce moisture loss to a minimum. Bonsai need less water when they are dormant, but the pot should never be bone dry. If your tree is not getting snowmelt or rainwater, water every 3-4 weeks.
DONTS: Don't put an outdoor tree outside in the middle of winter if it has been growing indoors. The shock will kill it. Autumn's cooler temps help to harden the trees and prepare them for the winter haul. This is why we keep outdoor plants outside, especially at night, leading up to winter. Autumn is when the tree's sap/sugars retreat to the root system to help protect them. Don't water your outdoor bonsai if the temps are going to drop below 32°F that night. The pot should drain a good 36 hours, after which the pot won't crack if it freezes again. Just check the weather report.
Water is crucial for your plant to stay upright, metabolize, and to stay cool. Without enough water, pressure inside the stems drops, and the leaves will wilt. Water helps in transpiration, and in cooling the plant. Your plant knows how to adjust a little to its conditions, but remember that there is no single watering schedule throughout the year. In high summer, when the light is strongest, your plant will require more water; in winter, your plant will go dormant and require less. A good rule of thumb is to never let your bonsai get too dry. Feel the weight of the pot in your hands after a good watering. The pot should drain quickly, but it will still be heavy. Pick it up the next day -- when it starts to get light again, it is time to water. The pot should never feel too light. This means the soil inside is very dry, and your plant will not survive long in these conditions. On average, a medium sized pot will stay humid for several days indoors. If outside, it will need daily watering.
“I’m on vacation and can’t find a plant-sitter!”
In summer we all need to take a break, but your bonsai will thank you if you give it a good soak before leaving. If you are away for more than a week, you can place your bonsai in a shallow dish with about 1/2” of water. This will humidify the air and allow water to come up through the bottom of the pot. It should be fine like this for up to 10 days, but take it out once you are back and resume your normal watering routine.
Sunlight allows your plant to create the food it needs to put out new growth and store energy for winter. Each plant has their own requirements. Full sunlight means 3-6 hours/day of direct or very bright conditions. Partial sunlight means at least 3 hours of direct sun, and partial shade is anything from 1-3 hours. Southern facing windows, barring any close buildings or trees, will usually receive the most daily energy from the sun. East and west facing windows receive morning and evening sun, respectively. Morning sun is great for many of our partial shade plants, being more gentle on delicate leaves. If indoors, place your plants close enough to the window to receive plenty of light, usually within 5 feet. Larger windows of course will cast greater light further into the room. It is fine to move your plant around your home as necessary. If you want to place it in a windowless bathroom to keep you company, that’s fine, but remember not to leave it there for days. All of our plants like to be outside, too. If you don’t have a balcony or outdoor space, crack open the window to give them fresh air. Take them on a field trip!
Good air circulation is important for cell growth. A slight breeze will stimulate the leaves and branches, encouraging roots to become stronger. It also helps to prevent the growth of molds by evaporating any pooling water quicker, and prevents dust buildup on the leaves.
Most of our bonsai are outdoor bonsai, meaning they go dormant in the colder months of winter. Displaying your bonsai indoors is fine, 2 weeks or less, but they will need outdoor conditions to remain healthy. Opening the door or window and placing the bonsai on the windowsill will help, if you have no outdoor space.
In winter, keeping your bonsai indoors in a warm room during January and February is like staying awake for days -- you just want to rest. Our bonsai can tolerate freezing temperatures and can stay outdoors or in a cold room during the winter. Find a protected area that stays between 25° and 45° F. Protect the plant from strong winter winds by placing the pot in a box, planter, or other container without a lid. If inside a cold room like a storage room, placing your bonsai in a box gives it extra protection from getting damaged or knocked over. Your plant should not need much water or light, as it won’t be growing, but you should check up on it anyway. When the pot is very dry, give it a full soak of water. This protects the roots, helps the plant put out new growth in spring, and prevents the leaves (on conifers) from drying out. Check the plant in early spring for any new growth, and when the nightly temperatures are above 32°F, bring it back into the open. Only start bringing the bonsai into your warmer living space when there isn’t wide fluctuations in temperature between indoors and out.