Invasive plants

I don’t know its name. I am guessing it is a species.

Remember this pretty aster I showed in March? I loved its profusion and lightness. So did the bees and butterflies love it. But I was worried that it was too large and too dominant immediately by the path so set out to reduce its bulk and spread. That was illuminating. The initial divisions that I had planted were now so dense I had to cut them into squares to get them out but that was fine. What worried me was how far and how fast it was s p r e a d I n g. It had probably colonised its way out about a metre all around by sending out long runners below ground. The runners will put up a small rosette of leaves maybe every 20cm but they keep running and they are certainly aren’t going to let the roots of other plants deter them. Nor indeed were the stone edging or compacted path a deterrent.

That is about two years of growth in the pink tub and on the path and just from one patch.

By the time I came to the second clump, I decided on total elimination and that turned into quite a major operation. The first clump, I just tried to reduce and thin somewhat but I am pretty sure I may have to carry out more drastic action next year.

I am trying some in a confined drum and will see how that works. It is such a lovely plant in leaf and flower that I want to keep some of it but I really don’t want it invading large areas. Come to think of it, it may be the same aster that took over the Missouri Meadow at Wisley, choking out most of the other plants.

We inherited several old tanks with holes in the bottom that I am now using to contain plants that look dangerous

We are extremely cautious about invasive plants in our garden. Make that our country, not just our own garden, because garden escapes of invasive plants are a major problem in the wild. I like to shock overseas gardeners by telling them that it is actually illegal to grow the giant gunnera where we are in Taranaki. It became a noxious weed on the coastal cliff tops near Opunake where eradication was such a huge issue it was banned altogether. Our soft, warm-temperate climate makes plants which may be called ‘vigorous’ or ‘strong growing’ in harsher climates downright invasive pests here. And it is not just gunneras. Agapanthus, flag iris, campanulata cherries, the bangalow palm (Archontophenix cunninghamiana) , erigeron daisy, pampas grass, perennial sweet peas, Fuchsia boliviana  – our country is littered with plants that are a great deal more prized overseas but either discouraged, banned from sale or banned entirely in some areas here.

My gardening friend from Christchurch, Robyn Kilty recommended Calamgrostis ‘Overdam’ as a possible alternative to C. ‘Karl Foerster’ which had become dangerously strong growing here. I looked it up and found I can indeed buy it but the description from one producer included the terrifying words ‘may be invasive’. Make that ‘will be invasive’ in our conditions. Where Robyn gardens in Christchurch, such plants are not a problem with their cold winters and hot dry summers which restrict growth. There is no such brake on their growth here.

Some plants invade by seeding too freely. To some extent, that can be controlled by dead heading if the plants are lower growers. We dead head most of our agapanthus here and I dead head the likes of crocosmias. Or choosing sterile varieties can eliminate the seeding problems, especially when it comes to trees.

This white daisy – name unknown – is vigorous without being invasive. If cut back hard, it flowers a second and even third time which is very obliging of it.

Some plants invade by putting out runners along the ground which then make roots. Wisteria are a prime example of this and believe me, you do not want to plant wisteria unless you are willing to restrict them and prune them at least once a year. Somebody once told me that the largest plant in the world is a wisteria which has layered its way along. I have no idea if that is true but I wouldn’t be surprised.

And some plants, like the aster (and indeed the calamgrostis) invade by determinedly spreading their roots below the ground. They are way more problematic though you can resort to spraying with herbicide if you use it. The underground spreaders tend to be very strong indeed, choking out the competition and getting their roots intertwined with anything in their way.

I have kept six plants of Calamagrostis ‘Karl Foerster’ to see if I can contain them by root pruning them once a year with a sharp spade. I think I discarded 18 of them and digging those out was a major effort, I tell you, after only one year in the ground. I had to barrow in quite a few loads of soil to fill the holes that were left.

Mark is going to try some of the aster down in the park meadow where the competition from other plants is greater and the area is mown twice a year so that may curb the rampant growth. If it doesn’t, he will have to reach for the herbicide.

A profusion of aster that looked as though it had the potential to turn into a takeover by aster

19 thoughts on “Invasive plants

  1. Pat Webster

    I know I should be sympathetic to your problems with invasives but all I can do is moan. And be a slight bit envious. I would love to have an aster that spread vigorously. Or to have wisteria that survived. But in my cold climate, that just doesn’t happen. So we learn to love — or at least to accept — conditions that cannot be changed. I hope the aster in the park meadow will work well and won’t overwhelm its companions. It is beautiful to imagine it blooming there.

  2. Ann Mackay

    Wow! We have some plants than can be invasive in our garden, but nothing remotely like the problems you have! Now I feel grateful that our ‘thugs’ aren’t as bad.

  3. Paddy Tobin

    We have some asters which also can become a pest, growing too strongly and spreading too freely. I am inclined to dump them immediately as leaving them would certainly lead to bigger problems in future years.

      1. Paddy Tobin

        Ah, the intelligent plant which deceives you with its pretty appearance while quietly conquering your garden!

      2. Abbie Jury Post author

        This does not happen in the cultivated areas of the garden. Best not ask about the wisteria in the wild garden, though.

  4. tonytomeo

    Invasiveness must be much more of a concern where invasive plants have potential to naturalize into the surrounding ecosystem. Where I lived in town, I did not mind invasive plants so much. They could not get far anyway. In this region, I want nothing to escape into the forest that is not there already.

      1. tonytomeo

        I suspect that the ecosystems of New Zealand are more sensitive to invasion than the ecosystems here are, because they have always been isolated from everything else. Ours are sensitive too, but are attached to the rest of the continent, from which some species migrate to expand their natural range. Some of that is not considered to be invasion, since it is natural, but does make ecosystems a bit more resilient.

      2. Abbie Jury Post author

        That horse has bolted here, Tony. But that is no reason to add more noxious weeds to what has already escaped into the natural environment.

      3. tonytomeo

        Well, exactly.
        Are there species of Yucca there yet? Some were considered to be safe if imported to Australia without their very exclusive pollinators, but are somehow getting pollinated and out out control.

      4. Abbie Jury Post author

        Yucca are not an issue here, as far as I know. But the drier areas of NZ tend to have cold winters. The rest of the country has too high a rainfall for plants like yuccas to thrive. Wildling pines, however, are a massive issue in the wilderness areas.

      5. tonytomeo

        Yucca that are endemic to desert and chaparral do not mind chill. Yucca glauca is native as far north as Alberta. Yucca that are endemic to tropical jungle regions do not mind rain, but are not the sort that have naturalized in Australia. Yucca are probably not allowed in New Zealand now, although anything can be sent by mail.

      6. Abbie Jury Post author

        Anything that is already on the country and on the official list is allowed to be imported again – subject to phytosanitary inspections. It is anything that is not registered as being here that has to have a very expensive and lengthy risk assessment done. There is quite a lot of mail interception done by border control. I have no idea how much is sneaked in. It used to be a lot but we don’t often hear about it these days. Easier by far, we decided a long time ago, to play by the rules. The reputational damage of being caught smuggling plants in such a small country is huge.

      7. tonytomeo

        The potential for environmental damage is too much of a risk too. I wish it were something that were taken more seriously here, even though it is not as risky.
        I brought a station wagon full of blue spruce from Oregon because I was told that it was not a problem. It was only not a problem because the unqualified people I inquired about it did not know what a blue spruce is. They either did not see it on the list of quarantined plants, or did not care. I asked about them again when I came into California, and the kid at the station just asked “Are they cane berries?” I told him that they were blue spruce. He asked “Are they elderberries?” I told him that they were still blue spruce. He asked “Are they roses?” . . . Well, it continued on like this for a long time before he sent me on through because there were other cars waiting to get in. I found later that blue spruce were not allowed into California from Oregon at that time.

      8. Abbie Jury Post author

        We are lucky that we do not have internal borders like the US and Australia. And, being an island nation, our international borders are few in number and generally staffed by people who have skills in determining what plant material is permitted. Well, at least not of that level of sheer ignorance!

      9. tonytomeo

        Species that are native to the islands have likely migrated to everyplace they want to go by now, so would not exactly be ‘exotic’ if put somewhere else on the same islands. However, islands a more susceptible to invasion by exotic species (that are not native to the islands) because they have not contended with them before.

      10. Abbie Jury Post author

        Our definition of a native is an organism that arrived here of its accord. So coconut palms up north are new natives (washed ashore from the Pacific Islands and taken root). Same with a certain amount of life that has blown or flown over from Australia. And I understand the monarch butterfly turned up, blown off course, all on its own. But yes, on islands that were generally pristine until fairly recent times (particularly mid 1800s) and with no indigenous fauna, deliberate introductions have had a high impact.

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