Knowing when to adjust the feeding and watering of your hibiscus is one of the most important skills a good grower needs to learn. Southern California is unique because in many areas we are able to keep our favorite plants outside year round thanks to the Pacific Ocean and it’s ability to moderate our annual temperatures. Because of this factor our plants can experience the changes in the seasons even when it doesn’t always feel like the seasons change a whole lot here. In fact you would be surprised to see how much your hibiscus know what time of year it is and they will start to make changes even when the weather and temperatures still seem to be perfect for them. We have observed every year in November that the blooms start to diminish in size and the splendid color displays of summer start become less intense. Now don’t get discouraged because some cultivars’ blooms look their best in cooler conditions so it just depends on which hibiscus plants you have.Climate Zone Legend
Before we get into what to do with fertilizing your plants in winter let’s cover a few basics of taking care of your hibiscus in winter. As stated above the Pacific Ocean is the great moderator of our climate here in Southern California. This means as you get further inland from the ocean that moderating effect diminishes and you become more vulnerable to the continental air masses of winter. This means that if your average annual low temperatures get near or below freezing on occasion during the cool months your hibiscus need to be in pots and placed either inside or under a substantial protective covering at least during those stretches if not for the entire cool months. Hibiscus are tropical plants and have no way to protect themselves from frost of freezing temperatures.
Another big problem for hibiscus is our cold and super dry wind events we get frequently from November thru March. Hibiscus are bred to live in constant warmth and high humidity year round as their origins are from tropical locations. So the wintertime Santa Ana wind events are the exact opposite of what they need. Worst case scenario is the plants can actually get wind burn and quickly dry out. What compounds this situation is when it is also cold their metabolism significantly slows down so they are very slow at getting nutrients and water back into all the branches and leaves of the plant. This will result in further plant decline to the point where it can go into catastrophic shock and either shut down and go barren or die. So if you live inland and are prone to those cold and dry wind events your hibiscus should be in pots and moved to a sheltered location away from the direct force of the winds. An example would be if the winds blow into the north side of your property you would want to place your plants on the south side and up against something like your home that will shield them from the full force of the winds.
Another important basic is that if you can every time your hibiscus get water they should also be fed. The only exception to this is when the temperatures are over 105F and we will talk about why that is the case later on in this article. Hibiscus have tender root systems that are susceptible to root rot. Root rot happens when the soil your hibiscus is growing in becomes too saturated with water and there is not enough air left in it. Hibiscus originate from tropical volcanic climates where the soil is usually rocky and porous. Tropical climates usually mean intense rain for short periods of time so hibiscus are used to a lot of water that quickly drains past it’s root system and then it is back to a soil environment that has lots of air in it. Add to this that volcanic soil is full of minerals such as iron and sulfur and they have developed into plants that are dependent on a high and readily available amounts of nutrients for them to greedily uptake. For many of us here in Southern California we have clay soils which are pretty much the opposite of volcanic porous soils so it is critical that if you grow your hibiscus in the ground you dig very large holes or trenches and replace the native soil with a soil mix that is heavy in perlite and/or pumice stone so that the roots of your plants have a very well draining environment. We recommend to dig a hole at least 1 ft down and 1 ft diameter around your plant at a minimum. Shadier spots the holes should be larger if you have the space to do so to increase drainage and the area that water can disperse over.
Root rot is at it’s worst in the cooler months. Those bad pathogens flourish the most and multiply the fastest in an environment that has no oxygen and is cold. So wintertime your plants are most at risk for root rot but ironically we see many of our growers get root rot in the summer and fall. That sounds surprising at first but the answer is simple. When the weather is hot and dry people tend to water their plants a lot at the first sign the soil looks dry. That is the mistake, the soil looks dry at the surface but many times if you were to dig deep down to where the roots are chances are it is still wet if not very wet. So unknowingly many growers add way too much water during those hot stretches and instead of lightly watering the surface to keep that layer moist they are saturating the soil deep down eliminating the much needed air. A simple way to avoid this is to have a water meter that is long enough to probe all the way down to the bottom of your pots. We recommend to slowly probe into your soil so you can watch your meter and see where the moisture level starts to change from the dry top. If you see that towards the bottom it is very wet then you want to sprinkle just enough water on the soil to moisten that top layer only. You have to become an artist with a delicate and masterful touch!
Also remember root rot is an infection inside your plant. It is contagious just like humans spreading diseases to each other. So if you have a plant that has root rot or died of root rot you need to disinfect your pot, any tools used handling them, throw away all soil too as it is now full of the pathogens that infected your plant and will just reinfect any new healthy plants you replace it with. For existing plants you need to very gently bare root them and wash the roots with a 10% bleach solution, cutting off all rotted roots. Make sure to disinfect your sheers after each cut as you can reinfect your plant. Once done repot in fresh soil after you have disinfected your pot. We recommend disinfecting your pots with pure bleach so as to not take any chances with surviving pathogens.
There is one last item to consider for potted hibiscus. Never rest easy when you think you have the right soil mix and a good draining pot. You will be amazed how quickly hibiscus roots will plug up all your drainage holes and before you know it you are getting root rot. An example on the extreme end is I have 9 gallon ceramic pots and I drilled 7-8 additional large holes on the bottom of my pots and then added plastic furniture coasters on the bottom to raise the pots off the ground to ensure great drainage and lots of air circulating under the pot all the time. Well wouldn’t you know it within 1 yr some of those plants started to get root rot. Turns out those extra vigorous Hidden Valley Hibiscus root systems had plugged up all the holes and that is in a 9 gallon pot. We advise to watch our video here on our website by Brad Daniels about how to root prune. It is a skill and action all good hibiscus growers will need to learn and do.
Now on to fertilizing your hibiscus in winter. The rest of the year minus heat waves just follow the instructions on the fertilizer label. We highly recommend using Hidden Valley Hibiscus Special Blend Fertilizer as it is formulated for exotic hibiscus. That would be one teaspoon per gallon of water.
So as stated at the beginning of this article you should always be feeding your exotic hibiscus year round. There are two parts to this answer to consider. First is how much do you feed your exotic hibiscus during certain times of the year? That depends on the weather and temperature. Your plant absorbs nutrients and water based on the heat it is exposed to. So the hotter it gets the quicker it’s metabolism becomes and the more water and nutrients it will intake. When temps get over 95F you will have to start reducing the amount of ferts your plants get since their metabolism is speeding up a lot. What was once fine for your plants might start to burn them since they are up taking at a much faster rate and thus more fertilizer is being absorbed too. So I start to reduce the amount of ferts by 1/4 for every 5 degrees over 95F. If the temps get over 105F I don’t feed them any ferts and just give them water. That is very rare and might only happen a few days a year where I live. So this is really the only time you will want to water your plants without feeding them.
For the colder months your plant’s metabolism will slow down. I have observed along with HVH that the 50F barrier seems to be a good one to watch for. Once night temps get under that you will see your plants start to really slow down. If the nightly temps get under 40F they will slow down almost to a crawl for their metabolism. So what this means is that they are not up taking much water and thus much fertilizer. If you continue to feed them at regular amounts like you would in spring and summer what happens is that those fertilizers start to build up in the soil of your plant’s rootball since it is not up taking that much. That can lead to severe fert burn when the plant goes back to regular metabolism now that your soil is full of unused fertilizer. So you would do the same thing as when it is hot and reduce the amounts of ferts. REMEMBER THE GOLDEN RULE – what you see now happened 2-3 weeks ago. So as the temps start to warm up from the winter lows it will take your plants 2-3 weeks to start to rebound and speed up their metabolism. So you need to be patient and wait for that lag time before you start to increase their ferts again. We advise keeping a log, journal or some sort of tracking mechanism of the weather, temps and inputs you gave so that you will best gauge when and how much to gradually increase for your plants.
This brings up another important factor – when your plant’s metabolism slows down it also takes longer for that lag time to pass. The colder the nights the slower your plant gets which means that it could take longer than the 2-3 weeks to see the change back to more normal metabolic rates. For instance if the night time temps get into the mid 30s that will slow your plant down to an extremely slow metabolic rate to the point where it might take 4-5 weeks for it to rebound once the weather pattern finally shifts and warmer weather starts up again. Getting under the 40F marker is a good one to roughly mark when your plant’s metabolism will take longer than the usual 2-3 weeks of lag time to show effects. Same is true in summer but the opposite. When temps get over 100F the lag time is closer to 1-1.5 weeks. When over 105F it is around 5-7 days.
Don’t get too excited when you see the new spring growth. Remember the golden rule and let that guide you.
The last factor to keep in mind when feeding your plants in the cooler months is how wet the soil is. We have had quite a bit of rain the last several weeks here so even though the night temps have only gotten down to the mid 40Fs I have not fed my plants for over 2 weeks just because the soil is way too wet. So they would like to be fed but if I did that would be placing too much water in already wet soil. One positive effect from lots of rain is it will also wash down excess ferts past the rootball zone for in ground plants so that can help offset too much fertilizer sitting in your soil. And that rain also helps to wash off your plants and all the dust and things that accumulate on them. If your plants are in a protected area from the rain than you’re good to feed them. Just track the nightly low temps to know how much to feed them. Once the temps get under 50F you will want to start reducing the amount of ferts. I would use the same rule of thumb as for when it is hot. Every 5 degrees reduce the amount you are feeding them by 1/4.
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