These culture notes were written by Jim Purdie and refer to the growing conditions in Brisbane in Queensland, Australia. People in the Northern Hemisphere should adjust the timing to suit their climatic conditions.
We have prepared some notes on the program which we follow for the 12 months here at our place, starting in August when the spring weather is around the corner, right through to the following July.
Also there is notes on growing in pots, and a picture and our method of dealing with Erinose Mite.
In August I apply Dolomite to my ground, to raise the PH up to the desired level of 6.2 to 6.5, which is the desired level of ph for the plants to grow at their best, if the ground is too acid, the elements in the fertilizer, are locked up in the soil, and the plant cannot take them up sufficiently, and the growth suffers. I use Dolomite because it contains magnesium as well as calcium. The reason I apply it in august, is because it takes about 6 weeks to take full affect, and is best not to apply fertilizer at the same time you apply dolomite, which you will have to do in the spring.
This is the month that we here in Brisbane start to prune, although you will have to watch in the colder states, to make sure that any danger of frosts have passed. Take notice which plants are tall strong growers, and those which are slow low growers. The tall growers can be cut back by about half, while the slow growers usually about one third. Cut about 1/2 an inch above an outward pointing eye, on a slant away from the eye.
Clean out any old wood and open up the middle of the bush by removing any branches that are filling up the space in the middle, and any spindly branches that will not produce a good growing limbs. As the weather warms up, the plants will start to send out new shoots, and this is the time to apply a fertilizer high in nitrogen, to promote the new growth. Keep on applying the high nitrogen fertilizer, until the plant looks like it is starting to get to the flowering stage, which is usually getting towards November, and then start to apply a fertilizer higher in potash than nitrogen, to promote the bloom growth. If you keep on applying the high nitrogen, you will get lovely lush green bushes, but no flowers, you need the potash to get the blooms and to add size and colour to the blooms.
In September I usually mix up a fertilizer to put on the beds, consisting of the following- I use a 2 litre ice cream can as a measure, 2 cans of Blood & Bone, 2 cans of Super Phosphate, 1 can of sulphate of Potash, 1/2 a can of Magnesium Sulphate[ Epsom Salts ], and a 1/4 can Sulphate of Iron. Mix this well together, and apply a good handful around each bush. When the plants are ready to flower, I change over to a fertilizer called Hydro Complex, which has a higher content of Potash than Nitrogen, but if you have only a few bushes, continue to use a high Nitrogen fertilizer, and get yourself some Sulphate of Potash, and apply about a good handful of high Nitrogen in say the beginning of the month to each bush and a dessertspoonful of Potash in the middle of the month to give the extra potash to promote flowering, I am talking about an established bush here, if you have only a small plant you have just planted a month or so ago, I would only use a teaspoon full of each until the bush gets established. Always water the ground before applying fertilizer, and then again after you have put the fertilizer on, to wash the fertilizer in, to avoid burning the roots.
I always apply a good covering of mulch after I prune, this keeps the weeds at bay, conserves moisture, the roots love to grow just under the covering of the mulch, in the cool moist ground, and also the mulch gradually breaks down to provide a good humus to the ground which the worms just love. I have used sugar cane mulch, composted grass and leaves, pine bark fines, Ti tree mulch, any thing will do, as long as you use some of kind of covering to keep the ground cool and stop the surface from drying out, as hibiscus have surface roots, and if the ground surface continually dries out you lose these roots, and the plant suffers.
November - December - January
Usually the weather here is very good for flowering, and up until the first couple of weeks in January, so just keep applying the fertilizer each month, and applying the water. Keep a look out for any insect attack, and deal with it when ever you see any bugs. I usually start to see the little black beetle around this time, and I use white 2 litre ice cream cans, white seems to work the best as the beetles are attracted to the colour more than other colours, in which I put a couple of inches of water and some cheap washing up detergent, and put these around the garden out in the sun where the black beetles can see them, I have about 24 of these scattered around my yard, and when the beetles fly over they think it is a big white flower and they land in the water and drown, you must use the detergent to break the surface of the water, so they will sink, otherwise they swim across the surface and crawl out. Also make sure that you pick up the fallen buds and the spent flowers each evening, to help in the control of the Black Beetle, as they breed in the ground, and if you do not pick them up, you have a factory going on producing dozens of beetles. Put the buds and blooms in a plastic bag and put in the rubbish bin, some people even spray a few squirts of insect spray to kill the beetles in the bag.
February - March
From the last weeks of January, through the months of February, March, we do not get many flowers, as the weather here gets too hot, and the plants throw off their buds, and the insects breed in the hot humid weather, causing bud drop as well. Watch the water in the hot weather as they use a lot of water due to the evaporation through the leaves, another cause of bud drop, and water in the cool of the evening, and if it is really hot it pays to hose the plants in the middle of the day as well. Bud drop happens whenever the plant is under stress, that is one way they show they are not happy. Pick up all the fallen buds each day. Still keep up the fertilizing with the high potash.
April - May - June
These are the best months for blooms here, so keep up the fertilizing with the high potash, at least every month, and enjoy the fruits of your labours.
July - August
These are our coldest months, and I do not use a solid fertilize on the ground, as the sap flow in the plant slows down in the cold weather, and the plant does not take up its nourishment from the soil, so I always apply a liquid fertilizer to the leaves at this time of the year, and this supplies the plants with all the nourishment they require. I apply this liquid mixture by the way of a fertigator. I mix up the solution in a 50 litre rubbish bin and the fertigator goes into my hose about 1/2 way along, by using hose connectors, and when I turn on the hose the fertigator sucks the fertilizer up out of the bin at the rate of 10 to 1. So I water my plants & fertilize at the same time, but I always do this before lunch, so as to give the plants a chance to dry off before the cold night air comes in, otherwise the leaves can get burnt with the cold or fungus sets in. And then we are back to the start of my year where I put on some more Dolomite.
I often get people ringing up, worried about yellow leaves. This is something that happens periodically throughout the year. It is a natural thing that happens with hibiscus. It can happen after we get rain when the plant goes into a burst of new growth, after you fertilize and the plant gets new growth, or if you spray for insects sometimes causes yellow leaves, but what happens is that a new leaf will start to grow and all the energy goes into the new growth, so the old leaf does not get any nourishment, so it begins to die, and going yellow is part of the leaf dying, but the new leaf soon takes it place and the plant looks nice and green again. Also it can happen at the beginning of winter when the sap flow slows down, there is not enough food for the old leaves, and only the top green leaves are left, but as soon as you prune the bush new growth starts to appear again. The main thing is do not worry if you get some yellow leaves. You may a problem if the whole plant goes yellow and starts to look very sick looking, then you may have a root rot problem, and it is sometimes hard to save a plant in this condition.
Growing in Pots
When you grow hibiscus in pots, you must use a potting mix that is very open to allow good drainage, as hibiscus do not like growing in too wet a mixture around the roots, I mix a potting mix of my own here, which consists of 50% composted pine bark fines, 30% compost, 10% garden soil, 10% coarse river sand, to which I add some dolomite to bring the ph up to 6.2-6.5, and some blood & bone.
You have to watch the watering in the summer as the pots will dry out very quickly, and the pots get very hot in the sun, so I water every day, and even in the winter you have to watch, as the winds can dry pots out very quickly. I do not apply the same fertilizer that I give my garden plants, as there is a danger that you will burn the roots with solid fertilizer, I use osmocote 9 months in the mixture, as well as soluble fertilizer on the leaves. When you start off I use a 6 inch pot to plant the smaller plants in, and then gradually work up into the next size which is a 8 inch, and so on up. After they are in the larger pot that I wish to keep them in, after 12 months, in the spring, I will scratch off as much of the dirt from the top of the pot, even to digging out some of the top roots, if I cannot get down far enough, so I can refill the top of the pot with some fresh potting mix, and this will carry the plant over until the next year. After 2 years it is a good idea to remove the plant from the pot and prune off some of the roots and at the same time prune the top branches in proportion to what you take off the bottom roots, and shake off as much of the old mix from the roots, and repot in the same size pot with some fresh potting mix. I grow the plants in pots in full sun, but you have to fertilize more often than you do the ones in the ground, as when you water the fertilizer is washed out of the mixture fairly quickly, so you need to fertilize at least every week.
Above is a picture of Erinose Mite, so as you know what it looks like if you come across it in your garden.
It is a microscopic insect that you have to look at it with a powerful microscope to be able to see it, and it floats in on the wind and when it lands on your plant it burrows into the leaf and causes the lumps which you see in the picture.
When you find the lumps take that leaf off, or the branch if it is on a whole branch, and put it in a plastic bag and dispatch to the rubbish bin, if you do not remove it when it is only a small patch , it will spread through the sap system up to the tip of the branch, and gradually move down through the whole bush, once it is in the main sap system it is very hard to remove. I find that by feeding and watering the plant, the plant seems to be able to outgrow the problem, as long as you remove the affected parts before it gets a good hold on. The DPI recommends spraying with Grub Kill at the rate of 22mills to 7 litres of water, but I do not bother to spray as I find removing the infected parts before they can spread works ok for me, and you do not know when some more will float in on the wind, and you would be spraying all the time in case some arrived.