Tag Archives: invasive plants

When good plants go feral

While I like the smaller flower and the airiness of the plant, the aster must go

I have spent the better part of the last week digging out an invasive aster. Pretty it may be in flower but I am now aiming at total eradication from the twin borders and I am looking at in askance in the Court Garden.

I don’t know what the species is but I am pretty sure it is a species, not a named hybrid. I see it was back in 2020 when I first decided it could be a problem. I lifted masses of it but left a few bits behind to see if it could stay with closer management. I would have started with just one small pot of it but from modest beginnings and despite some intervention along the way, I could now measure it by square metres. It has to go.

Veritable thickets of aster in the borders

The problem with this aster is not just that it needs staking in our climate because it reaches maybe 1.3 metres high in flower and flops over under its own weight, smothering everything around it. I could have coped with some seasonal staking and restricting its spread from seed by deadheading it, but it is what is happening just below the surface that is frankly alarming. It runs in every direction with huge enthusiasm. I am lifting it out of areas where I never planted, several metres away from the original clumps.

Surface rooting but it is those runners that are the real problem. They will put up shoots along the length and just keep running in every direction.

Because it is shallow rooted, I can lift the mats of roots okay and often just grab each runner in turn and carefully pull it out for most of its length – which can be 30cm of runner at a time. The problem is where it has invaded the root systems of its neighbours and the runners break off. It is going to take me years to achieve total eradication from the borders.

The Missouri Meadow at Wisley in 2009 – simply magical
and in 2017 – the Missouri Meadow choked by aster
That Missouri Meadow aster looks very similar to the one I am eradicating, although we were there too early to see any more than just the occasional flower

At the back of my mind is what happened to the Missouri Meadow at the RHS flagship garden Wisley, in the UK. When we first saw the meadow in 2009, it was in its second year and remains in our memory as one of the most enchanting plantings we have ever seen. By, 2017 it was a real mess because – wait for it – an invasive blue aster had taken hold and run wild. I would guess the whole garden has long gone now and been replaced by something else because getting rid of just the aster would have been impossible. I don’t know if it was the same aster but I could see that, left unchecked, my twin borders would have gone the way of the Missouri Meadow in just a few more years.

We have maybe half a dozen other named cultivars of asters in shades of blue and pink and they are fine. They are all hybrids, not species, and while they will form a fairly dense surface mat of roots, they are not invasive in the same way. And they do tend to have stronger stems so hold themselves upright, even the ones that are waist height.

Aster novi-belgii ‘Professor Anton Von Kippenberg’ is more of a compact bedding plant and I have used it in the Wave Garden. It needs digging and dividing regularly but it doesn’t keep trying to stage an escape, trampling its neighbours in the process

Also going from the borders are all the Miscanthus ‘Morning Light’. This is not because they are invasive, although they do seed a little more than Mark thinks they do. I know this because it is I who weeds out the seedlings. Beautiful they may be, especially in flower in late autumn as the sun drops in the sky and illuminates the plumes in the lowering light levels. But they are relatively demanding and, if not managed tightly, they grow too dense and fall apart in heavy rain and wind. Basically, they need to be lifted and divided every three years and root-pruned in the intervening years and that is a big job in densely planted borders.

Alas the Miscanthus ‘Morning Light’ must also go from the borders

I have used the same miscanthus extensively in the Court Garden and they can stay there, even though the lot need to be lifted and divided this winter. That can be done when the flowering is past its peak. But it is another lesson we learned from several visits to Wisley that led to my decision to get them out of the borders next to the Court Garden. It seemed to us that Wisley had used Stipa gigantea in almost every garden there and, while a most obliging plant with the prettiest flowers of any grass I know (golden oats, is its common name), when it is used in many garden beds, it just ends up making them all look the same. Too much of a good thing. I have generally avoided repeating plants in the different summer gardens so that each one has a different look. Miscanthus ‘Morning Light’ was one of the very few plants I repeated and, with hindsight, that was not a good decision in our growing conditions as far as the twin borders go.

So I have spaces to fill but plenty of other material to use that will be better. The thing about gardening is that it is a constant learning process but that is also what keeps it interesting. A stitch in time may indeed save nine when it comes to sewing; in the garden, removing certain plants in time may avert a takeover that will choke everything else out.

Ralph is no respecter of gardens, especially as this border is adjacent to The Rabbit Family Who Live Beneath the Swimming Pool Deck and he is on their case every day.

Elegia capenis

Elegia capensis in the new Court Garden

I lost count of the number of times we were asked about Elegia capensis during our recent open days. I have used it extensively with the grasses in the new Court Garden and it is looking rather splendid.

Its common name is the horsetail restio and the entire restionaceae plant family hails from South Africa. There are plenty of them but E. capensis  is the most common in ornamental gardens. It is not too fussy on conditions as long as it never dries out entirely and is not subjected to very cold winters.

Most visitors described it as looking like a bamboo but with feathery growth, which is apt. The new shoots come up in spring like narrow bamboo shoots, first developing brownish sheaths at points up the stem before growing the fine foliage in tiers. The stems last about three years before dying off.

When you look at the seed head, it is just as well that seed is not viable here

Our plants have never set viable seed. Whether that is because they are all one clone and they need other clones to set seed or whether it is the lack of smoke from bushfires, we are not sure. They may even be fully dioecious – needing both male and female plants to set viable seed. These are plants that have evolved to deal with frequent fires (they sprout afresh from underground rhizomes). PlantzAfrica says: “The seeds react well to treatment with smoke or with the ‘Instant Smoke Plus’ seed primer. Without this treatment the germination rate is poor.” I have never even heard of Instant Smoke Plus before.

We used to produce a few to sell in the nursery but the death rate in production was high. I have only just discovered why. We assumed that they would be similar to other plants like hostas and rodgersias that grow from rhizomes. In other words, we would lift the plants in winter, separate the rhizomes and repot them ready for spring growth. Then I read somewhere that they should be divided in February – late summer – so we tried that and it was not particularly successful either. What I have since discovered is that their roots are very sensitive, making the plants difficult to propagate by division. They are evergreen and not overly hardy so they are in growth all the time, unlike plants like the aforementioned hostas which have a dormant period. Commercially, they are more commonly raised from seed.

Where we have been successful in dividing this elegia, it has not been by taking apart the rhizomes but by cutting off large chunks of the original plant and putting the entire chunk into the new location. When I say cutting off large chunks, this requires a strong person, a very sharp spade and sometimes an axe, to take off blocks that are about 20 cm across which need to be replanted straight away. Brute strength, not high-level skill.

I see BBC Gardeners’ World describe this plant as invasive, which I think is wrong. It doesn’t seed readily – or at all here. Nor does it run below the soil surface so it is not invading. It is, however, strong growing and can make a large clump in the right conditions and that large clump is not easy to reduce or eliminate because of the solid nature of the rhizomes.

The maceaya is the poppy foliage amongst the elegia

What is setting out to be genuinely invasive is the Macleaya cordata, commonly known as the Plume Poppy which is interplanted with the elegia. We have it in a shade garden where it certainly ‘ran’ below the soil surface but not in a particularly problematic manner. In full sun in the Court Garden, it is not so much running as sprinting – in every direction including into the pristine new paths. Attractive it may be, but it is a worry. I may just have to leave it in the shade garden and find a less determined plant option for the sunny, grass garden.

Terry’s restio, as we refer to it here.

We have another restio that I picked up from Terry Hatch at Joy Plants in Drury but I am not sure where – or if – I kept its species name. It is not as vigorous as the elegia so better suited to the perennial borders where I have it planted but lacks the immediate visual charm of the deep green colour and tiers of feathery foliage up the stem. There is no such thing as a perfect plant for all situations.

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