This is my new mantra. We have always had some level of self-seeding in the garden, some more desirable than others. We are fine with desirable plants like Scadoxus multiflorus ssp katherinae and the Himalyan lily, Cardiocrinum giganteum, deciding where they are happiest growing and gently settling in by spreading seed; we call that naturalising. Similarly, when the plants are natives, we don’t worry about them spreading around and we just decide whether we want them where they appear and weed them out if not. Nikau palms, tree ferns, kowhai and the like fit into that category.

Then there are the invaders, of whom our worst offender is Prunus campanulata. At the moment the parent plants have a stay of execution but we aim to remove every seedling we see. We cut down the parent bangalow palms (Archontophoenix cunninghamiana) because of their rampant seeding. It may be we reach a day when the prunus suffer the same fate.

Establishing the summer gardens has added a whole new range of plants that want to expand their territory beyond their allotted area – not for nothing are they called colonisers. The reason is that plants growing in well-cultivated soil in sunny conditions can multiply and spread exponentially, both from below ground and by airborne seed.  

When I planted the summer gardens in the years between 2017 and 2019, I made it a priority to use different plant material in each garden. While they are adjacent to each other, I wanted clear differentiation between the Wave Garden, lily border, Court Garden, back border and the twin borders while anything and everything that was left over was fine in the wilder Iolanthe Garden. I avoided repeating plants because as soon as your start using the same plant palette, they all end up losing definition and looking the same. I was pretty disciplined about it and there are only one or two repeats.

Many of the plants have different ideas and that is escalating. Which is why I find myself saying often, “It is a good plant but we don’t need it everywhere.” I mentioned  mondo grass in last week’s post. It is very handy, both in black or green but it also sneaks all round the place both by seed and by spreading below ground. Too much. We will have much too much mondo if we don’t take charge now. Besides, it doesn’t look that great when it gets very thick and it is not easy to thin without a major effort on lifting, dividing and replanting. It needs restricting and reducing right now.

But there are so many more spreaders. Perennial lobelias, tigridias, verbascums be they chaixii or virgatum, Verbena bonariensis, crocosmia, scuttellaria, lychnis, gaura (so much gaura…), gladiolus and more. I have been gently restricting Gloriosa superba but I now think I have been too gentle and I need to get them out by the barrow-load and confine them to just two or three areas. And white foxgloves. I went to quite a lot of trouble to get a pure strain of white foxgloves established but now they want to be everywhere. Knowledgeable visitors from Europe last spring were astounded by the size of my white foxgloves, declaring that they have never seen them so large in Europe.  Therein lies the problem: foxgloves are thugs that smother their neighbours and they seed far and wide. They need to be kept to a limited number of places where they can star but not smother.

Where we are, there is little to no danger of these plants escaping into the wild but I can see how we have arrived at a situation in this country where many, possibly most, of our environmental weeds are escapees from gardens. Pretty escapees they may be, but it does not stop them being weeds when they spread uncontrollably in the wild.

We gardeners need to take responsibility for our plants.

I do a lot more deadheading these days than I ever anticipated but that is basically short-circuiting what will be an even larger task if I don’t. A stitch in time and all that. We don’t generally compost seed heads. While we make hot compost, it is not always hot enough to kill off seeds and we certainly don’t want to be spreading viable seed right through the garden in the compost. Mostly, we dump them in piles in deep shade out of sight, which is possible in a large garden but not always practical in small, urban gardens.

The retired nurserywoman in me feels bad about wasting so much good plant material that has commercial value, but not to us. When we had the nursery, we mainly grew woody trees and shrubs which generally take a whole lot more skill, specialised facilities and time to get to saleable size. We used to question how some perennials could be sold at a similar price to a shrub or tree. Now I have a whole lot more experience gardening with sunny perennials and realise how easy it is to multiply most of them, I am even more amazed at the prices I see in garden centres.

One of the interesting parts of gardening is deciding plant combinations and it is certainly easier when you have too much plant material rather than too little so that, at least, is a good problem to have. Verbena virgatum can go entirely though. It is way too enthusiastic and if I turn my back it will have infiltrated everywhere. It is pretty enough but not a good plant when it comes to behaviour. Keep to Verbascum creticum, I say. Similar large yellow flowers although in spring, not summer, and while it seeds, it has never been a problem for us.