I have a large infestation of DSV and have started digging it out. The question that I have is this: what can I plant in the large empty areas, 3-4 square metres each, to inhibit or prevent the re-growth of DSV? The areas are mostly sunny and open with some conifer trees and dogwoods. One area has a steep west-facing hill. Also in these areas are buckthorns … a daunting job in itself!
Oh your are brave! Digging out dog strangling vine (Cynanchum nigrum) or black swallowwort as it’s sometimes called, is a hard job as it is very aggressive in terms of underground rooting or stolons. Once you have all the DSV out (or hope you do) the next step really is not to plant. Digging out the DSV has disturbed the soil and raised up all kinds of seeds from other weeds that will now start growing. I would recommend further amelioration by laying down a black, opaque polyethelene plastic sheeting over the open ground, and holding it down with rocks or bricks to keep it from blowing away. The black plastic does several things: It stops germination of any new weed seeds as they have no light or water, as rainfall will be blocked from entering the soil too. This will also be the case for any of the left over rootlets from the DSV. Further, the black plastic will attract heat from the sun which will effectively cook any seeds or roots underneath it – this is called solarization. It is important to keep the cover on for at least 6 weeks and the best months to do this are June, July and August as they are the hottest. Be vigilant in watching for any re-growth of the DSV as you may not have killed off every last root. If you see some popping up, just take a secateurs or sissors and cut the vine off just below the surface of the soil – no need to start digging again. Do not let it leaf out as you don’t want it to start photosynthasizing and making food – you want to starve the already weakened root in the ground so that it runs out of energy for any growth.
After that, soil amendment will likely be required, as you have dug up the organic layer, DSV not-with-standing. It might be prudent to order a yard bag from a reputable supplier of compost. They all deliver which makes it more convenient. So, if you get a yard bag (1mx1mx1m), you can wheelbarrow compost to where you need it and mix it into the soil. If your soil is sandy, you can add black earth to enhance the crumble factor. The plants that come later will love it.
It would be a challenge just to plant perennials to prevent regrowth in these large areas. I would recommend ground cover shrubs, such as a number of the spreading junipers (Juniperus horizontalis), such as Blue Rug, Turquoise Spreader, Prince of Wales, to name a few. All spread to over a meter, low to the ground and are dense enough when mature, to prevent a lot of weed invasion. Other evergreens that spread include Cotoneaster (Cotoneaster dammeri) which has small winter foliage, blooms in spring and produces colourful berries. Euonymus fortunei, a favourite with many for their hardiness, will spread too, keep leaves over winter and come in a variety of variegated foliages such as Emerald and Gold, Gold Tip, Blondy and Emerald Gaiety. You might even want to consider the wilder versions of roses which bloom and smell sweet. Memorial Rose (Rosa wichuriana), is a prostrate rose and is particularly suitable for covering slopes. It has white flowers and red rosehips in the fall. The other shrubs noted would also do well on the hill.
The European buckthorn (Rhamnus cathartica) also is an invasive pest with nasty thorns. The black fruit are favoured by birds which is primarily how this plant spreads far and wide, especially along old hedgerows and fencelines. They also drop their fruit and young plants spring up soon after so they are just as much of a challenge as the DSV. I would remove them and replace them with more suitable native trees, such as hawthorn (Crataegus sp.) or cherry (Prunus sp.), or if you have the space, oak (Quercus sp.). Treat the soil the same way as the other cleared areas before planting anything new.