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Ask an expert: Layer of cardboard can be used as weed barrier - with caveats - oregonlive.com

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Cardboard can be used as a weed barrier in the garden, but it has certain disadvantages. File photo of Oregon.

Q: I plan to change about 550 square feet of land in my yard from grass to rocky paths with bushes, flowers and bark. I was instructed to lay down a layer of cardboard and then put about 4 inches of bark or mulch on the first step. Is this correct (or at least a good way)? We will gradually plant, but there is no time (or money) to complete all the work at once. --Multnomah County

Answer: You may be surprised to find that this is a controversial technology! First of all, I will comment that I have used this method many times and have achieved good results. This is especially effective for leisure routes, because many of the problems I will mention later are not problems.

To use this method, there are some key things to keep in mind. First, completely cover the area, overlap the cardboard by 8 to 12 inches, and block any gaps created by the lid. The grass is spread by rhizomes, and it is great in finding a way to the light. Put a few inches of organic matter on the cardboard to compact it and complete the seal. You can use wood chips, leaves, compost, straw-no matter how much you want to have, you want to add to the area. If you want to increase the area, you can also use soil.

Seasonal timing is also important. If you cover the grass that is entering the dry season, it will go into a dormant state and be ready to act when the rain starts next fall. If you cover it in autumn or winter and make sure to soak the cardboard and mulch, the grass will be covered when it just starts to grow and will die. By next fall, everything will be better.

For garden beds, the answer is somewhat subtle. Dr. Linda Chalker-Scott of Washington State University opposed the study on the basis of research. The study showed that the lack of circulation of air into the soil and the subsequent reduction of soil life, which is of course a bad thing. You can read her report here

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In addition to her worries, for many years, even the dead turf was still a thick, impenetrable obstacle for excavation and other plant roots. Therefore, I don't recommend using this method for any bed that you want to plant quickly, unless you plan to do the whole thing (after determining whether the turf is good or bad). Remember, farming will bring many weed seeds buried in the ground.

This is also more problematic for vegetable gardens or perennial borders where small plants are used. However, for shrub/tree borders, the plants will be large and well-spaced, which can be very useful. Make sure that the grass is dead before planting, and according to Linda's research, I also recommend using a fork to penetrate the cardboard in many places after the grass dies, so that the oxygen circulation can be restored more quickly. – Signe Danler, OSE promotion horticulturist

The date of the last frost varies by location. File photo (AP Photo/Olin Wagner)

Q: I live 9 miles east of Sandy (889 feet above sea level). How do I find the average last frost day in my area to estimate when to plant seeds to move the plants into the garden later. --Clackamas County

Answer: Generally speaking, the frost date in our area is around mid-April. I recommend that you use a soil thermometer before planting seedlings outdoors. Use the information on the seed pack to get plant-specific details. Please also check out this article:

There are plenty of detailed resources in your area. You can also consult the USDA website, but due to outdated documents, my results are mixed. – Shawn Van Doren, Oregon State University Extension Gardener

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Q: Which material is most suitable for the construction of elevated garden beds? Are concrete blocks toxic to growing food? --Josephine County

Answer: First of all, this is a

On the raised bed garden of OSU Extension, their possibilities are mentioned.

Secondly, this is a

, This said for concrete blocks: “Cement blocks, cinder blocks and concrete blocks are all made of cement and fine aggregates (such as sand or small stones). Fly ash is also often included. Fly ash is a by-product of burning coal and therefore contains heavy metals and other hazardous waste. The label does not provide exact information about the aggregate used in the manufacture of the block. There is also very little research data on this topic. In the end, this will be you Personal choice of comfort level. If you plan to use the block as a raised bed material (many people do) and you are concerned about the potential risks, you can seal the block with polymer paint. Or, you can choose to use other materials .-OSU Promotion Master Gardener Kris LaMar

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