A plant that grows where you do not want it is called a weed. A plant that grows, by itself, where you want it is called a volunteer. Whereas volunteers are normally not relied on in conventional gardening, volunteers are the basis of bush gardening. In fact 80% of the indigenous plant species at Wirrapunga came as volunteers.
There is no limit as to how long seeds can remain viable within the soil waiting for the right triggers to tell them it is time. As well there are a number of ways that plant seeds may travel. Birds and animals transport the seeds in a number of ways. Some seeds are blown by the winds. Orchid seeds may be blown hundreds of kilometres. Volunteers can turn up unexpectedly at any time. A good bush gardener is always waiting and ready for them.
Of course one can always assist the process by collecting seeds from desirable species and scattering them about your bush garden. It is always a surprise to find where they decide to grow. It is normally not where you expected.
The first necessity to make best use of volunteers, is to recognise the plants in your garden. Just as the conventional gardener recognises the plants in their garden so must the bush gardener. It is probably worse to remove a plant that is needed as to not remove one that isn't.
It is helpful to be able to recognise juvenile plants but this is not necessary. There are two basic rules. Firstly, do not remove a plant unless you are sure you do not want it. Secondly, do not let a plant set and disperse seed if you do not want it. This means that a plant may be left until it has flowered for the first time. The plant must then be identifies as friend or foe - volunteer or weed - and left to disperse its seed or removed before its seeds are dispersed.
Once a volunteer turns up in your garden it is important that it is given the necessary attention so it may multiply. I find it useful to place a marker of some sort near it so I can keep an eye on it. Carefully weeding out adjacent competition is normally enough to allow it to multiply and spread. If the plant is uncommon then it is a good idea to
try to get a couple of more patches growing by placing some seed from the existing plants in other suitable positions.