Tag Archives: invasive plants

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