The Invasion of the space snatchers
Recently , I heard a woman complain that the leaf mold mulch available from the city is full of invasive weeds, so she didn't want to use it in her raised beds.
I have a couple of comments on that.
First, any soil component or growing medium that does not have weeds is sterile, that is, there is no life in it. Lifeless soil will not in any way add to the health of either soil or plants. It is merely a support structure. Without input in the form of fertilizer, the plants will not thrive. They may not even survive.
Second, invasive is a very over- and misused word. An invasive plant is one which has invaded a space, and - left unchecked - will smother all other plants. It is necessarily a non-native. Some plants, when transplanted (in the metaphorical sense) make do with the new environment, some do very poorly, and others say, whooppee, this is for me. Kudzu comes to mind. Or autumn olive. Or bush honeysuckle.
The better behaved plants can be considered immigrants rather than invaders. The invaders proliferate by several means, often more than one. Seeds, obviously, but also bulbs, rhizomes and runners above or below ground. (David Attenborough did an amazing time lapse shot of raspberries taking over a forest floor.) Less aggressive plants that spread by runners are known as ground covers. Without strict supervision, such plants can occupy even quite a large area. Think bugle, lysimachia, vinca, pachysandra. The difference is that they do not jump to another area: they can be contained, at least in the garden. (Vinca has escaped and can be quite a problem in uncultivated areas.)
The pictures above show two immigrants, one of which is - one could say - a naturalized citizen, and one an invader. Can you guess which is which? (answers upside down at the bottom of the page! Or would be if I knew how to do that) They are: from the top Lesser Celandine or Fig Buttercup (ranunculus ficaria), forsythia x intermedia, and primroses (primula vulgaris).
Primroses are very well-behaved immigrants from Europe; Forsythia is from Asia, but has been hybridized (intermarried) to such an extent that one can consider it a native. The charmingly pretty ranunculus (also known as ficaria verna) is one of the most aggressive invasive species we have. Introduced from Europe/West Asia, it can be a major problem.
I mistakenly identified it as Marsh Marigold, a somewhat rare native, and was happy to see the mounds of hairless green leaves topped by buttercups appearing in the early spring. It is somewhat ephemeral (gone by June) so I didn't see it as much of a problem. Furthermore, I have been somewhat lazy in the garden, and now the problem is huge.
The photo is just a small part of my invaded garden. I am pulling it up as fast as I can. Fortunately, there are still leaves that I have not yet raked off the beds, so I am building an enormous compost pile. If the weather co-operates just a smidgen, I should have a good hot pile going in the next few days.
Who says you can't make you a silk purse out of a sow's ear, so to speak.