The most effective way to inhibit weeds is to plant something in their place. Weeds will grow on any open ground. But most weed seeds will not sprout on densely shaded ground. The best plants to shade the ground are groundcovers, shrubs & spreading perennials. Weeds are less of a problem in shade. But seeds will still sprout on lightly shaded open ground.
I don’t want to discourage you. But there will always be weeds. You must be vigilant: Pull weeds before they set seed! Weeds come from seeds. Plastic, rocks, gravel & bark will not keep weeds away forever. Weeds mostly seed in from above. Bark & other mulch will help to keep weeds under control for a time, while other plants have a chance to get established. Be sure you can distinguish weeds from the groundcovers you have planted.
To get rid of weeds that already exist, you should probably just pull them out. It will help if you loosen the soil with a garden fork, hoe or hook. Don’t be fooled into thinking that is the end of the weeds. There are undoubtedly still seeds in the ground. Keep watching for new weed sprouts.
Weeds with long, fleshy roots (like dandelion & bindweed) will grow back, if part of the root is left in the ground. This is also true for blackberry. It can be very difficult to get all of the roots of a blackberry vine. If it grows back, keep cutting it off at ground level. I don’t think that herbicide is any more effective than cutting.
Here is a list of groundcovers that grow in shade for garden in Seattle, the Pacific Northwest & USDA Zone 8. Generally, groundcovers are perennials that spread over the ground by one means or another. Groundcovers are very helpful to control weeds. They give depth & richness to a garden. They make plantings look more natural. Choose several groundcovers if your garden is small, many if your garden is large. A single groundcover will spread throughout, looking a bit weedy. As in business, it is better to give groundcovers competition than to let them monopolize your garden. You will also get better coverage & weed-suppression by using a variety of groundcovers. The groundcovers pictured above are all Pacific Northwest native plants. Many sources of information will tell you they require moist shade. But these will all survive the summer with very little irrigation. I've seen Achlys triphylla growing luxuriantly in dry Ponderosa Pine forest on the eastern slopes of the Cascade Mountains. Although they grow in shade, do not use Hedera helix (English Ivy) or Euphorbia amygdaloides robbiae (Mrs. Robb’s Bonnet). They are too aggressive, will overwhelm other plants & grow out of control. English Ivy is considered a noxious weed by the State of Washington.
Groundcovers for Shade
Achlys triphylla (Vanilla Leaf): tolerates dryness,spreads widely, but not densely by rhizomes Ajuga reptans (Carpet Bugle): tolerates some dryness, many cultivars, spreads by stolons & seed, also grows in sun
Here is a list of groundcovers that grow in sun for garden in Seattle, the Pacific Northwest & USDA Zone 8. Generally, groundcovers are perennials that spread over the ground by one means or another. The exceptions here are some woody trailing or vining plants & some low spreading shrubs. Groundcovers are very helpful to control weeds. They give depth & richness to a garden. They make plantings look more natural. Choose several groundcovers if your garden is small, many if your garden is large. A single groundcover will spread throughout, looking a bit weedy. As in business, it is better to give groundcovers competition than to let them monopolize your garden. You will also get better coverage & weed-suppression by using a variety of groundcovers. Most of the groundcovers listed below tolerate dryness. Xeric plants prefer dryness & often require well-drained soil.
Here is a question sent to me in June 2009 about rototilling weeds.
We're starting to get serious about landscaping our backyard. Time and money have been the main reasons we've been unable to since we moved in. Since we've moved in, the area has been taken over by weeds. I tried to work on removing them, but it's turning out to be a lot of back breaking work!!! I went to home depot and they notified me that rototilling it would be my best bet. but they mentioned to rototill the area, and then leave whatever's been tilled there, including the weeds. I understand that this will turn into a good compost for the ground? If we have plans to eventually lay out patio blocks for a sitting area, should I keep the tillings there still? And for the areas that we'd like to just have rocks, or maybe bark, should we also leave them there? As you can tell, I'm pretty novice to this, so any advice would be greatly appreciated.
It may take a while for you to control your weeds. You need to remove the roots from the area you have rototilled. Many weeds can regenerate from just a small piece of root. If the weeds have set & dispersed seeds, which they probably have, you will have many new weeds growing in the area. Rototilling will not destroy weed seeds, it will merely move them around. You will have to pull weeds, by 1 means or another, for 6 months or a year. Weeds do not make good compost because they are full of seeds.
If you lay out blocks for a patio, you will likely have weeds growing in the cracks. These will come both from seeds remaining from the rototilling & from new seeds blowing in from the surrounding area. Rototilled ground is loose & will settle. It is much better to lay paving materials on hard-packed earth. Otherwise, your blocks will sink over time, settling unevenly. It would be better to scrape off the top layer of soil with the weeds in it, or pull the weeds. If you have particularly pernicious weeds in your yard, like blackberry, don't pave over them. They have deep & extensive roots. They will definitely come up through the paving.
I'm sorry to tell you that weeding is forever, even in areas with bark or rocks, although bark does help for awhile. The best defense against weeds is to plant shrubs & groundcovers. Most weed seeds will not sprout on densely shaded ground.
This blog began in 2008 as Metropolitan Gardens to provide information on gardening in Seattle & places like it. Click on Gardening in Cascadia. The blog expanded to include Parks P-Patches Public Gardens in the US, Canada & Europe. Then Nature was added. (The difference between nature & gardens is intent. Nature happens. Gardens are planned.) Many recent posts are not about Seattle, or even Cascadia. They are Urban Landscape: streetscape & architecture. Cascadia is the region including the Cascade Mountains of Washington & Oregon. If you want to contact the blogger, leave a message in comments. Comments are moderated & private messages will not be posted.
The city of Seattle rests between 2 bodies of water: Puget Sound & Lake Washington. Puget Sound is a substantial part of the Salish Sea & a very small part of the Pacific Ocean. The Salish Sea is set apart from the Pacific by the Olympic Peninsula in the state of Washington & Vancouver Island in the province of British Columbia. The dense, wet clouds of the Pacific Ocean travel as far as the Cascade Mountains, near the Salish Sea & not very far from the ocean. East of the Cascades lies the desert of the Columbia Basin. The moist, temperate climate of Seattle extends south to northern California & north to southeastern Alaska. The Pacific Northwest Coast from San Francisco Bay to Cook Inlet shares a flora dominated by evergreen coniferous forest. The central portion, west of the Cascade Mountains, is called Cascadia. The climate is cool & wet from fall to spring, warm & dry in summer. The Olympic Mountains block Seattle from much of the Pacific rainfall. Seattle is drier than the Atlantic coast of North America & northern Europe, cooler in summer & warmer in winter. It lies near the latitude of Paris & Quebec City.