Division is a term loosely applied to a number of propagation methods where a plant is split into two or more pieces all of which have roots attached. "Division" means exactly what it says. At its most vigorous it means driving a spade through the centre of a clump, pulling one half of the plant out of the ground and planting it somewhere else in the garden. Provided adequate water is provided, it's surprising how many plants will tolerate this sort of barbarism! Not surprisingly, though, a higher success rate can be achieved with a more gentle approach.
Division is most often used with plants which form grass-like clumps. Apart from the grasses themselves, these include Kangaroo Paws (Anigozanthos, Macropidia) and lilies (eg. Patersonia, Orthrosanthus). Other candidates for division are plants that sucker and those which naturally layer (ie develop roots where their branches touch the ground. Suckering plants include some species of Dampiera, Lechenaultia and Scaevola while those that self layer include species of Hibbertia and Isotoma.
Nothing elaborate is required; secateurs (a spade or shovel for the bigger clumps!), a sharp knife, pots of various sizes, potting mix and, most importantly, somewhere to put the divisions while they develop a strong root system. This doesn't need to be a dedicated greenhouse, a position sheltered from wind and direct sun will do for many species. Even better is a few pieces of wood nailed together to support a clear sheet of plastic ...a sort of make-shift greenhouse. It might not look professional but the plants won't mind!
"Grassy clumps" generally have strap-like leaves arising from an underground rhizome which is a thickened underground stem. The best known Australian plants in this group are the Kangaroo Paws. Less well known are Dianella and Stypandra, both with spikes of small blue flowers, and Lomandra.
The technique involved in this group is quite simple. Once the clump is dug up (or tipped out of its pot), it's divided into segments that comprise a piece of the rhizome, some leaf shoots and some roots. Dead leaves and dead roots should be removed. If you just want a few plants for your own use, the best sized division comprises three or four leaf shoots with a good section of rhizome and healthy roots. With these, the humid environment of a greenhouse or similar shelter is probably not required, just a sheltered position and regular watering.
If a humid greenhouse environment is available, the divisions can be quite small, comprising only one set of leaf shoots. Once potted into individual small pots, these are kept in the greenhouse for a few weeks (4 to 6 weeks in the warm part of the year; longer at other times). It may be beneficial to trim the leaves to reduce moisture loss through transpiration. Divisions of this size will also need to be "hardened" after the roots have developed. A simple method is to expose them to the open air during the night for a week or so, putting them back into the humid shelter during the day. After this period they can be moved to an outdoor, sheltered position for at least another week before potting on.
There are a host of plants which spread by producing suckers from their root systems. Some of these are so vigorous and invasive that propagation is probably the last thing on the gardener's mind. Bamboo, for instance. Others, however, are less anti-social.
Among the Australian natives Dampiera and the related Lechenaultia often spread by suckering as do some Lobelias and Peperomias. Even seemingly unlikely genera such as Grevillea and Acacia have occasional suckering species.
All you need do with this group is to dig up small pieces (making sure that there are roots attached), pot them into individual containers and place them into a greenhouse or sheltered position for a few weeks.
Layering is a propagation technique (not used all that often these days) where a branch of a shrub is pegged down, covered with soil and then severed from the parent plant when roots have formed. There's a bit more to it than that, of course, but in some cases, the plants do all of the work for us!
Most of these "self-layerers" are ground covers where branches are in contact with the ground due to the growth habit of the plants. Several Hibbertia species are in this category. H.pedunculata, for example, often forms roots at many points along its branches. Propagation is simply a matter of lifting the branch, cutting it into segments (each containing some roots), potting the segments to small pots and treating them in the same way described for the "grassy clumps".
Another group of plants which fall generally into the layering category are those that send out stolons (lateral, above ground stems which for roots at the leaf nodes). These usually form small plantlets where they touch the ground and these can be cut off and potted up. The well known exotic "Spider Plant" (Chlorophytum comosum) is a good example of this type of growth but many others (including some of the Australian Conostylis species) do much the same thing.
If you don't have access to a greenhouse (or don't feel inclined to construct a temporary structure) you can successfully propagate layering species in-situ. Just place a small pot of mix under the branch and place the roots into it. If necessary, peg the branch down with some wire to prevent the roots being pulled out of the mix by the wind. After a few weeks the roots should be well established in the mix and the new plant can be severed from its parent.
The best time for dividing plants is variable and depends on the climate. In warmer areas, virtually any time of year is suitable. Without doubt, though, the best time is in the warmer months. Divisions taken in late summer should develop strong roots well before the onset of winter. Divisions taken in the cooler part of the year will often show little sign of development until the weather warms up. In very cold districts, little success is likely unless some form of greenhouse is available.