Growing Hibiscus In Containers
Tropical hibiscus adapt well to container growing because of their shallow rooting systems and container plants are especially versatile in confined areas. For people who move house a lot or for apartment dwellers who garden on balconies, potted tropical hibiscus can be exhibited at their best and at other times removed to obscurity. For those who live in very cold climates, containers are indispensable.
Types of containers
Select containers that are as wide as they are deep, or even wider. Width is necessary because the plant's fine feeder roots spread out from the base of the stem like spokes of a wheel, almost horizontally. There are more of these roots than there are anchoring ones growing downwards. A tall, narrow container is therefore unsuitable.
Plastic pots are fine, although in very warm climates they may heat up and harm the roots, and it is important to avoid one side of the pot heating up while the other shaded side stays cool. A safer arrangement in very hot areas might be to put plastic pots inside outer pots. Some professional hibiscus growers recommend clay-fired pots rather than plastic because unglazed terracotta pots "breathe". Containers made of stone or concrete can also be used. Timber too is an excellent material for accommodating hibiscus if it is the moisture-durable kind, like cedar, and wooden wine barrels, are wonderful.
While potted hibiscus may be heavy, their growth habit is gentle. Unlike vigorously rooted plants overdue for repotting or for planting out, hibiscus roots never burst their confines.
Orchids, fleshy-rooted agapanthus, shrubs, bulbous plants, and many others with expansive or exploratory roots can crack open a container. By contrast, when hibiscus roots reach their limit they start to suspend growth. As they reach the pot's perimeter they turn and grow down the sides before only slowly starting to fill up the center.
For a medium hibiscus (to 6 ft/2 m), a good-sized container to start with is one with a diameter at the top of 12-24 in (30-60 cm), tapering to a little narrower at the base, with the height of the container about the same as the width. You can grow hibiscus in pots wider than higher, but not the reverse, because a tall narrow size is wasted at the bottom and too restricting at the top, so start with a 12-16 in (30-40 cm) size, and after several years repot to a larger one.
Drainage holes should be in the outer rim of the base of the pot, rather than underneath - or both - because matted roots underneath can in time clog up drainage, and will certainly do so with pots standing in saucers -a good reason for raising pots on small blocks. Saucers have their uses in indoor culture and during times of absence when you are not there to water frequently. At such times shallow saucers can be left with water, which also helps cooling in excessively hot weather. In general though, since sharp drainage is essential, hibiscus grown in pots need side drainage and regular watering.
Because of the need for a mix with high air-filled porosity, your potting mix should always contain some sand. The best sand to use is "sharp" sand, which refers to the surface of the individual grains. Sand composed of sharp rather than rounded grains usually comes from granite and is better at providing air-filled porosity.
The mix must be friable and loose. A combination of 80 percent potting mix (usually fine pine bark) and 20 percent sand is quite standard; or you can use a 50-50 blend of high-quality garden loam and commercial potting mix with the sand. You can add a small quantity of well-aged compost to the potting mix (making sure it has no worms in it because worms block drainage), or a small scoop of fine peat, in addition to the sand. A soilless mix contains no trace elements, which can be supplied through slow or medium-release fertilizer. It is best if some compost is used in every mix, in small amounts only -or else applied as a surface mulch. Well-rotted manure can also be used as a covering mulch.
Always select compact and bushy hibiscus for growing in pots, avoiding especially plants that are scraggy, or tall and leggy. Although pruning can control size, it is best to buy a smaller bush at the start, preferably one with a thick branching habit and plentiful foliage.
Fill your container about a third full and gently position the hibiscus in place, taking care not to compress or damage the roots. Do not plant too deeply. Keep the join of roots and stem close to the surface. Add the rest of the mix to fill up the pot, then lightly pat down to leave a space of about 1-1 1/2 in (3-4 cm) at the top. Never fill right to the top. Water well. Once the plant has settled in the level may drop slightly and you can apply a thin layer of mulch. Mulching is beneficial but not essential.
Feeding and watering
If you use slow-release fertilizer in the form of coated granules or pellets, you may need to feed your hibiscus with these twice a year -once in spring and again in early fall. In a cool climate and with small plants, once a year may be sufficient. If you use a liquid foliar spray, feeding will be more often - once every two or three weeks during the growing season.
As for watering, water only when your hibiscus needs it. Scratch the surface to a depth of about 3/4-1 in (2-3 cm): if the potting mix is moist, don't water; if it is dry, give it a thorough hosing. Always water well after feeding.
Maintenance and pruning
The size of the pot controls the size of the plant. In a 12-24 in (30-60cm) diameter (and height) container, a hibiscus will grow to a maximum of 5 ft (1.5 m) high from the base of the pot, and will be about 39-51 in (1-1.3 m) across - which is probably as large as any gardener can handle. In fact, a smaller size may be preferred.
If you do prefer a smaller bush, just prune it. Not every gardener realizes that as you trim your foliage, your plant will self-prune its roots. It works like this. Suppose you reduce your shrub to about 20 in (50 cm). The reduction in foliage will trigger the retraction of the feeding roots down inside the soil. As they retract they leave tiny tunnels of air in the mix, like worm tunnels but narrower. The roots will react in this way whether it is a container plant or a large bush growing in the ground where the spread of feeding roots extends out to the leaf drip-line, so that the outer dimensions of the bush determine the outer limits of the roots.
During this time of root reduction it is imperative that you do not feed the hibiscus. The retracting roots are not capable of taking up nutrients; they are well provided for with air from the air tunnels they leave, while the plant at this time draws upon its own store of carbohydrates. So when radical pruning results in a surge of new growth, that growth comes from the stored energy of the plant, not from nutrients in the soil.
Trimming the bush to trim the roots is a wonderfully neat system for the home gardener. Although you can repeat the procedure as a reliable way of keeping a containered hibiscus in shape for years, it will eventually need fresh medium and rejuvenation. The color of the roots is a good indicator of general health: white roots are robust, while darker roots are often a sign of poor condition. When you repot, pot to the next size only.
You'll know when it is time to repot if the roots have grown through the drainage holes and the plant's appearance indicates that it has used up all the available nutrients and is ready for a fresh feed. Repot to the next size by removing -carefully -the root-bound plant and placing it in the potting mix in the new container with the stem join at the same level. Unlike other root-bound plants that respond well to having their matted roots teased out, hibiscus tolerate only very gentle handling of their roots.