For hibiscus lovers in the northern states, winter comes early, and so does the time to start figuring out how best to protect our hibiscus plants through the cold months. The most important consideration for tropical plants like hibiscus is staying warm in winter. Heat is more important than light or anything else, so let’s take a look at some good ways to provide heat to hibiscus in winter.
First and Foremost ~ Hygiene
First, before you do any moving, thoroughly wash your hibiscus. It is best to wash them several times before moving them inside, to make sure no stray pests have hopped onto them. Moving hibiscus is stressful to the plants, and any stress makes it harder for hibiscus to fight off pest attacks. Plus, indoor environments are cozy and wonderful for certain pests, like the obnoxious spider mite. So wash, wash, wash your plants to make sure you leave all pests outside. Spray every plant thoroughly with a strong spray of water, all sides of every leaf, stem, and branch. You don’t need soap or anything else, just water and lots of it. Wash your plants once or twice a week for 2-3 weeks before moving your plants inside. Then on the last washing, add horticultural oil to your water. It’s easiest to just pop a hose-end spray bottle onto your hose. Spray the plants heavily and thoroughly with the horticultural oil and water, then move them inside as soon as they dry on the last washing day.
Second ~ Don’t Stop Fertilizing!
If you stop fertilizing your hibiscus over the winter, they will go more deeply into dormancy and decline, and it will be much slower and more difficult to wake them up in the spring. It is very important to keep fertilizing through the winter months. If you use a Special-Blend Fertilizer, you will naturally water less in the winter, so you will naturally also use less fertilizer, which is perfect for your hibiscus. If you use a timed-release fertilizer, it will release more slowly in colder weather because the release rate is controlled by temperature. Your hibiscus is less actively growing and metabolizing in the winter, so they need less fertilizer, but they do need some fertilizer all winter long.
Overwintering in Cold Climates
Move them into the House
The place where hibiscus can stay warm without any extra cost is in your house. Although hibiscus is considered outdoor plants in the United States, in many parts of the world they are very popular houseplants, except for 2-3 months during summer when the pots may be set outdoors. Indoor hibiscus makes attractive, green houseplants. They help clean the air trapped inside in winter and give off extra oxygen. The greenery provides a wonderful backdrop to the drab and dreary weather of winter in many locations. And, every now and then, one of the hibiscus plants will bloom, providing a special indoor splash of color and beauty for that day.
Unless you live in Canada or northern Europe, you may not be accustomed to having hibiscus as houseplants, but they are easy to grow and quite suitable for indoor use. In the far north or any place where temperatures regularly fall below freezing during winter nights, tropical hibiscus will perish if left outdoors. Bringing them inside the house is an easy solution to overwintering them, but how best to do it?
The other main problem with placing hibiscus inside the house is finding space where there is adequate light. The closer to windows, the better for this purpose. But remember, hibiscus need warmth even more than they need light. So even if you only have room well away from windows, the hibiscus will do better there than if left outside or in an unheated garage. People have reported to us that directing just a little extra light to hibiscus in winter helps them stay green and healthy. A lamp placed nearby will help tremendously in a warm but rather dark area. What matters for hibiscus is the total amount of light they receive each day, so if the area tends to be dark you can leave a light on as much as 24 hours a day to help the hibiscus get what they need.
One of the potential problems is the size of the plants. Small hibiscus plants can grow very large by the end of summer. The simple solution to this is also good for the hibiscus – prune them back. You can reduce their size by as much as 50 percent without damaging them. More typically we cut the stems back by about 30 percent, but hibiscus is quite adaptable and will accept even severe pruning. So go ahead and reduce them to the size you need for them to fit well inside the house. Just be careful to leave some new growth and several older leaves on the plant after the pruning. Since it is winter and light are low, the hibiscus will grow back slowly, and should not grow large enough by winter’s end to become a problem in their indoor location. The idea is for them to get a head start on growing back during the slow growing season of winter, and then to rapidly grow to bloom size once they are placed outdoors in spring or early summer.
Garage, Shed, or Utility Room
If moving your hibiscus into the house is just not for you, then the next best strategy is to look for another solid structure where they can be kept over the winter. A garage, storage shed utility room, or any other structure that keeps out cold wind and provides some protection against the winter weather can be made to work. Again, the goal is to provide as much warmth as possible and at least some light to keep the hibiscus going. A structure with windows that allows sunlight to enter is ideal because the light will heat up the interior during the day and also provide the light energy needed by the hibiscus.
Even inside a shelter, in some colder climates, night-time temperatures can drop well below freezing. If possible place a small heater in the structure for use on those cold nights. Even better is a heater with a thermostat that you set to go on at 40°F (4°C) or higher. Small space heaters equipped with thermostats are usually inexpensive, so this doesn’t have to be a large expense. If you can afford to keep the structure as high as 65°F (19°C) the hibiscus will thrive. But with energy costs these days that much heat may not be affordable. The idea is to find the highest temperature you can afford to maintain, then set the thermostat to that temperature. Even 35°F (2°C) will help the hibiscus a great deal compared to allowing the temperature to fall below freezing.
Buying or building a greenhouse is the ultimate way to protect your hibiscus over the winter. These are wonderful additions to a property, and a good one will provide all sorts of opportunities to enjoy hibiscus year round. Whether you build your own or order one from one of the many greenhouse companies, such structures allow for year round growth and fresh flowers if heated to at least 60°F (15°C) during the winter.
One benefit of greenhouses is that they trap the rays of the early morning sun and heat the inside of the greenhouse far more rapidly than the outside air warms up. The greenhouse also reaches a much higher temperature during the day than the outside air. For instance, on a cold sunny day in a warmer climate zone, the temperature outside might be 35°F (2°C) at dawn, 50°F (10°C) by noon, and peak at 60°F (15°C) briefly in the afternoon. Compare that to the greenhouse temperature that, if unheated at night, will also be 35°F (2°C) at dawn, but reach 50°F (10°C) by 10 AM, 75°F (24°C) by noon, and peak over 80°F (26°C) during the afternoon. Similarly, the temperatures drop again rapidly at night outside, but in the greenhouse remain warmer for many hours before eventually matching the outside temps shortly before sunrise. With a heated greenhouse, the same thing happens but in addition to the warmth of trapping the suns rays in daytime, you also help the plants by keeping the temperatures at night at 60°F (15°C) or at whatever you set the thermostat for.
In a cold climate where heating costs are very high, most hibiscus can survive if temperatures stay at or above 35°F (2°C), but warmer is better. If you can afford to keep your greenhouse at or above 40°F (5°C), it will be much better for hibiscus. Every few degrees of extra warmth will mean healthier and happier hibiscus, so find the level that you can afford. Also, consider the duration of the cold temperatures. Sustained cold temperatures that last for many hours and/or many days or weeks will do more damage to hibiscus than an occasional dip into low temperatures for a short amount of time.
The ability to trap heat inside a greenhouse is so good that on warm sunny days the temperature will rise too high â€“ even over 100°F (38°C) on some days. You need to build your greenhouse with sufficient windows and doors so that you can allow heat to escape on sunny warm days by opening them to the outside air. The main downside to greenhouses is that they involve an investment of money and time to build. If you decide to make this investment, it will pay off in great fun and success with winter growing of hibiscus outdoors.
Hibiscus Blooming Indoors at Christmas
Some hibiscus lovers use their office space to grow hibiscus during the winter months. Or perhaps a relative or friend has a suitable space where you can winter your hibiscus. Avoid using a space that is too far from where you normally spend time, because the hibiscus will need to be checked on at regular intervals, mainly for watering.
Overwintering in Warmer Climates
If you live in a warmer climate, where temperatures rarely fall below freezing, and when they do it is only for one or two nights before warming up again, then you may be able to winter your hibiscus outside by providing some simple protection measures. If your area has a history of having fewer than ten nights per year that fall into the high 20’s (-2°C) or above and no nights colder than that, you have the possibility of protecting your hibiscus outdoors.
One of the best ways to protect outdoor hibiscus that is planted in the ground is to mulch over the root zone and around the main stem of the plant. A thick layer of leaves or compost can help protect the roots and keep them from freezing at night. This goes only so far though. For more protection, wrap the entire hibiscus bush in heavy frost cloth. This can add several degrees of freeze protection for the plants. In addition, you can run outdoor Christmas lights up under the frost cloth. During cold nights the lights can be turned on and the small amount of heat they give off will add several more degrees of warmth under the frost cloth. We have seen hibiscus bushes treated this way survive winters where temperatures reached as low as the 25°F (-4°C). A fully exposed hibiscus is usually severely damaged or killed by nights in the mid-20s, but just this amount of protection will prevent most frost damage.
Potted hibiscus in areas with just a few light freezes can be protected in other ways too. The pots can be moved up next to the house which will add a few degrees of warmth for them. A south or west wall with sun exposure during the day is another good place to locate the pots. Placing hibiscus under solid overhangs or under trees with thick canopies that prevent heat from radiating out into space at night also offer cold night protection. Some people report successful protection by tipping their potted hibiscus over on their sides and covering them with tarps or frost cloth on cold nights. Running Christmas lights under the tarps would afford even more protection.
Surviving an Unexpected Freeze
If your hibiscus is caught outside and unprotected by a sudden nighttime freeze, you can take action during the night to save them. This is a bad situation to be in but does happen occasionally even in the mildest climates. It happened to us three winters ago, and we used this strategy to save our hibiscus. We keep a temperature sensor outside in the hibiscus garden that radios the temperature to an indoor display so we can monitor the temperatures the hibiscus experience. This sounds exotic, but it is actually a simple device that is widely available at garden centers, costs well under $50, and runs on batteries.
One night three winters ago, we watched in dismay as the temperature dropped to freezing, then below freezing and showed no signs of stabilizing. By midnight it had dropped to 27°F (-3°C) and there was no way to protect our in-the-ground hibiscus. We had heard that simply running sprinklers on tropical plants could protect them from a freeze, and we decided to try it since we were sure our hibiscus would not make it through this very cold night without serious damage. So we went outside and turned on all our sprinklers. Soon all the hibiscus were being sprayed by the pop-up sprinkler system that was originally installed to water the lawn. How could this help, you ask? Well, the water coming up out of the pipes below the ground is considerably warmer than freezing. As this relatively warm water fell on the hibiscus and coated them, it protected them with a blanket of warm water. However, it was so cold that night that the water began to freeze on the leaves! We watched in horror as more and more ice built up on the plants, certain that we would lose all our hibiscus. Our sprinkler idea certainly looked disastrous, but it turned out to be protective. The continuous spray of water kept the ice temperature right at 32°F (0°C) and prevented it from dropping any lower. At 32°F (0°C) for only a few nighttime hours, the sap inside the hibiscus didn’t freeze. I left the sprinklers going until the air temperature rose above freezing the next morning and the ice melted off the hibiscus. Yes, we had a wet mess in the garden, and yes we wasted some water, but most of our hibiscus only sustained minor damage from that night and grew back the following summer as if they had never been through a hard freeze. We do not recommend this technique except in an emergency! But if you are ever faced with an unexpected hard freeze and unprotected hibiscus, a spray of water from sprinklers or hose will help keep the sap from freezing and prevent serious damage or death.
Older is Better
Tropical hibiscus can be grown and kept safe through winter, even in areas where freezing weather occurs. Remember, the main principle is to provide them with as much warmth as you can along with as much light as is practical in the warm area you place them in. The older the plants are the tougher they are in winter. We don’t worry at all about our 10-year-old and older hibiscus. Their thick woody main trunk and stems are not much affected by minor frosts and freezes and they are able to grow new shoots to replace any damaged ones each summer. The biggest challenge is with hibiscus experiencing their first winter. For these plants, we recommend an indoor, warm location whenever possible. If you provide the warmth, they will repay you with much beauty and enjoyment for years to come!