In 2009, I visited a Site of Special Scientific Interest fairly near to where I live called Pevensey Levels to look for the rare and unusual wildlife that is found along its ditches. What I found was this:

It is beds of Floating Pennywort Hydrocotyle ranunculoides, a plant native to the Americas and parts of Africa, but not to Europe and Africa.

You can see the effect it is having. It is growing so rampantly that it is outcompeting all the native ditch vegetation, and casting the water into a deep shade.

Floating Pennywort was first found on Pevensey Levels in 1995; by 2008 it was estimated to have invaded about 15km of the ditches.

So how did it get here? Well, no-one knows exactly, but we know in principle. It was brought to this country as a garden plant, and so either got washed out into the Pevensey ditches from someone's pond, or was dumped onto the Levels by someone. It needn't have been a malicious act; maybe it had outgrown a garden pond and the owner didn't like the thought of destroying the plant so thought it would be far happier in a bigger wetland.

Removing it? Now there's a challenge! It is very costly, and usually involves the use of pesticides, which as you can imagine is not what you want to use in a wetland.

Just like the Floating Pennywort, there is a range of plants that people have transported around the globe because they liked the look of them and wanted to grow them or sell them, but which have then leapt the garden fence and begun to steamroller across the countryside.

It shows that we all have a responsibility when growing plants to ensure that we take great care to not to grow the most damaging ones, and to ensure that any plant we do grow doesn't escape 'into the wild'. In fact, I find the term 'into the wild' interesting, because our gardens might feel 'tame' to us, but there is no barrier between them and the rest of the world; gardens are an interconnected part of our planet.

Taking responsibility starts with knowledge, so it is good to understand which plants are causing a problem. It is only a very small number of the 70,000 or so garden plants that we grow in the UK, but you might be surprised at some of them.

The most damaging of these invasive non-native plants are banned from sale, and that includes the Floating Pennywort.

But there is then what are called Schedule 9 plants. In the legal wording, "if any person plants or otherwise causes to grow in the wild any plant which is included in Part II of Schedule 9, he shall be guilty of an offence".

Plants in Part II of Schedule 9 include very common garden plants such as Montbretia, Gunnera tinctoria and Japanese Rose. Two Schedule 9 plants spring up everywhere in my garden as pernicious weeds: Few-flowered Leek and Three-cornered Leek.

Here is Schedule 9 Variegated Yellow Archangel:

And here is Schedule 9 Virginia Creeper:

And you might be surprised to learn that there are five Cotoneasters on the list, including the very familiar Wall Cotoneaster Cotoneaster horizontalis. Cotoneasters used to be such a widely promoted plant for wildlife-friendly gardening, but when you see them charging across a native downland slope, obliterating the ground flora that, for example, so many of our rarer butterflies rely on, you begin to see the issue.

Note that the law doesn't say that you can't grow Schedule 9 plants, but you may not allow them to escape. Personally, I now avoid them, and carefully remove them and dispose of them if I find them in my garden. Here is the full list of plants.

The problem is that some more plants not yet on Schedule 9 are beginning to show their triffid tendencies. Look at this photo I took on the Isles of Scilly.

It may look like a lovely bank of shrubs, but it is actually Wire Plant Muehlenbackia complexa from New Zealand, running riot. We are realising that what turn out to be invasive plants can often sit benignly in gardens for many decades before they suddenly gather steam and the problems appear.

There is an added and very worrying aspect to the invasive non-native species issue for gardeners, and that is the introduction of diseases and disease-carrying organisms that affect plants. The Xylella issue is just the most recent in an ever increasing list - this is a bacterial infection from the Americas that has now been brought to Europe and is devastating olive groves in parts of Italy, and is a risk to all sorts of plants.

As gardeners, we are at the forefront of these issues, so awareness followed by diligence is the key. Our love for plants must go hand-in-hand with caution and care for the impacts they may cause. This week is Invasive Species Week, and for more information on all these issues, check out the webpage of the Non-native Species Secretariat here.