It is important when we are choosing plants for our garden that are not endemic to our local area that we consider the potential for that plant to become a weed.  Once a weed spreads it can be near impossible to eradicate if it has had long enough to naturalise. In allot of cases the weed is happier in its new environment than where it is native to because there are limited to no herbivores and pests and diseases. It is expensive for local and state governments to eradicate serious weeds, and some can never be brought under full control causing permanent damage to the environment that increases exponentially each year. If you go to beautiful places like Bell’s rapids 30 mins North of Perth, you can see how bad the damaging the bulb weed Watsonia is and how it would be nearly impossible to bring it under control as there is not enough money and resources to allocate controlling such a widespread infestation. It has covered hundreds of square meters near the river, and it has probably done the same in many other similar areas around Perth. Another weed which is being brough under control somewhat is Arum Lily which has beautiful flowers, but it is a serious weed especially in Southwestern Western Australia in swampy/damp environments, but it can nearly spread anywhere. There are dedicated groups bringing this under control but if it is let spread in 1 paddock on a farm it can quickly spread into natural areas very quickly. 

Allot of weeds come from similar climates to where they are native to, and many South African plants have become bad weeds in Western Australia. However, some plants can come from tropical environments and become bad weeds in Western Australia particularly vines. We also need to consider wetland weeds like Hydrocotyl that are grown in garden ponds as they can spread fast and have serious environmental impacts. Even plants that are native to areas not that far away can even become weeds like Geraldton Wax has become a weed around Perth. 

Below are some plants that are or not that long ago were available in nurseries and have a high potential to go weedy or are serious environmental weeds in different states in Australia. 

Agapanthus – Agapanthus sp is a serious weed in Tasmania and NSW but is emerging as a weed in other states such as Western Australia, Victoria and South Australia. It spreads by seed and rhizomes (underground roots). 

Freesias are lovely perfumed seasonal bulbs that die of over summer in Western Australia but naturalise very quickly in a garden environment and quickly spread into surrounding areas. They are extremely hard to permanently eradicate once established as they grow from a corm. As beautiful as bulbs are I would be very careful and, in my opinion, most should be banned.  

Watsonia – Watsonia sp. also has corms and is a bad environmental weed in Western Australia. It has cormils to which it can spread from. It requires significant physical and chemical removal to control. 

Arum Lily – Zantedeschia aethiopica –  spreads by tubers and seeds and loves swampy paddocks but can naturalise to grow in most neighbouring environments. Physical and chemical control are required to control this weed. 

Veldt daisy – Osteospermum spp​​​​​​​ is an attractive and tough daisy that spreads rapidly in a garden and then will escape. Its growth rate is quite impressive, and you can have a garden full of it about 6 months after pulling out all the visible plants. This plant if it is not already banned should be. 

Alyssum – Lobularia maritima​​​​​​​ is related to Canola, Cauliflower, Broccoli etc and is edible also but spreads very quickly from seed in a garden with no water. It can easily spread to surrounding areas. 

Poppies or any of those tough exotic seed combinations are a bad idea and I have seen horticultural groups with the best intentions hand out these seeds, but I cringe knowing the potential they have to spread. With beautiful native annuals why not go to the effort to harvest and sell those such as Everlastings and Rottnest Island daisies for example.

Gazania – Gazania sp. is another South African daisy and spreads via seed and can get into sand dunes and surrounding areas but it is not fussy and will grow anywhere. It spreads quickly and control is possible, but it requires nasty chemicals to bring it under control. 

Dietes – South African Iris  – Apart from being tough this plant looks horrible when not in flower and I have no idea why you would not use Australia Irises such as Patersonia (Purple/Yellow flag) and Orthrosanthus (Morning Iris instead). It spreads rapidly from seeds as it is very efficient at producing seed capsules full of seed that has extremely high germination rates.  

Geraldton Wax – Chamelaucium uncinatum​​​​​​​ –  originates in Geraldton obviously but has been grown for cut flowers and exported over the world as the flowers look like they are made from wax and last a long time. They come in many colours and are quite attractive. This plant is very tough though and spreads readily from seed. 

Robinia Mop tops  – Robinia pseudoacacia – this naturally topiarised tree is a great plant if you hate your neighbours and want to make gardening as difficult for yourself as possible. It has sucker roots that will come up in your neighbour’s lawn, pavers and anywhere it feels like it, but you also get to watch them appear from places you thought were not possible. 

Lantana – Lantana camara – has become a bad weed in New South Wales and Queensland. Some varieties are weedy and are not available in nurseries anymore but other varieties are so I would still avoid those varieties. It produces heaps of flowers and hence heaps of seed and spreads easily from seed. 

Lavender – Lavandula sp  Is allot tougher than most people think and mature plants can produces allot of seed which has high germination rates. This plant can appear in lawns, roadsides to name a few locations I have seen it. As nice as it is make sure you keep it under control in your garden. 

Victorian tea tree – Leptospermum laevigatum – this has become a bad coastal and bushland weed in Perth and grows rapidly from seed. It grows to form dense stands where nothing else can grow. 

Olives have definite weed potential as mature plants produce many fruits which all contain seed with a high germination rate. Birds can eat the fruits and spread them, or people can discard the seed after eating them and new plants can grow from these seeds.