Keeping invasive self-seeding plants under control
The final months of the year are bought alive with the wonderful pastel shades of numerous asters. I have written about them before. Although some of them are perfect garden guests, some of them are real thugs, that will overtake a border in few years if you are not careful. Some will self-seed themselves, scattering seedlings across the garden in different colours and heights, whilst others produce invasive runners that can rapidly spread across a bed.
In our large border in the car park, a lovely clump of Aster novae-angliae ‘Violetta’ has been spreading across the border. I am very fond of its dark, intense colour, but the numerous seedlings in various shades varying from violet to pink have become a problem. We have marked the unwanted plants, so that we can remove them once flowering is over.
Asters are not the only ones that may seed themselves in your garden. Many plants have the potential of doing so, if your garden offers them just the right conditions they require to be germinate. Some plants may be perfectly well-behaved in one garden, whilst in the next they become a nightmare. Soil type, humidity and temperature are just some of the elements that need to be right for a seed to germinate.
With some plants I find it a compliment when they feel at home and gently spread themselves wherever they find a suitable place. They create a dynamic feel, altering the garden image from year to year. Many of these tend to be short-lived and will wonder through the garden from bed to bed. Evening Primrose is a plant that happily migrated through my Coleshill garden over the years, but never felt out of place. In my Berlin shady garden, it is the discreet but delightfully charming Montia sibirica that surprises me in different areas.
What to do about potential problem plants
Experience has taught me that when a plant threatens to get out of hand, it is up to me to take action before it becomes a serious problem. The simple solution is to dead-head the plant just after flowering, long before the seeds have had time to ripen. Lady’s Mantle (Alchemilla mollis) is such a plant which I always cut back just as the flowers start losing their fresh lime green colour, before they turn brown. Cut them back too late, they will seed all over the garden. Aquilegia and Lungwort are also plant I like to keep an eye on, as they cross-pollinate easily. This will result in unwanted seedlings that are different from the parent plant.
Dispose of the real threats
I am only prepared to invest this amount of work if the plant is one I really value. If you are not that attached to a plant, it is simply not worth the risk of it ruining your garden or causing a lot of unnecessary work. In such a case there is only a very radical solution: remove it completely. If you are a garden softie, no problem. Just dig it out, pop it into a pot and place it on your doorstep or by the garden gate with a small sign saying “I need a new home”. You will be surprised to see how fast this plant will have found a happy new owner!
Some of my favourite self-seeders:
- Digitalis purpurea
- Geranium pyrenaicum ‘Bill Wallace’
- Kentranthus ruber °
- Meconopsis cambrica
- Montia sibirica
- Oenothera biennis °
- Oenothera stricta ‘Sulphurea’ °
- Papaver somniferum °
- Potentilla recta var. sulphurea °
- Rudbeckia triloba °
- Stipa tenuissima °
- Tanacetum parthenium °
- Verbascum °
- Verbena bonariensis °
( ° these plants prefer a sunny position)