Written by Kimberly James.
The greatest labor input in vegetable gardening is weed control. There are many ways to manage weeds but one method that is gaining in popularity is using black plastic mulch. While plastic does significantly cut down on weeding time, you must also consider heat and water trade-offs before you decide if it is right for you.
Using black plastic warms the soil. This can really help in getting plants in earlier; however, it can also mean problems for cool season crops. We still opted to use it for our cole crops of broccoli, cauliflower, and cabbage but ended up mulching with straw over the plastic to try to cool the roots in the early warm temperatures. The other issue with heat we encountered was the super-heated air effect at the base of the plants. This was especially tough on the new transplants. The air between the plastic and the soil super-heats in the sun then escapes out the holes at the base of the plants literally cooking the tender leaves. To avoid this, we ended up using extra soil to hold down and seal the openings. We have not seen any major issues with seeds that were direct sown into the holes. To avoid or minimize this problem it is very important that your soil be well prepared without any large clumps before laying the plastic to avoid excess space and promote soil/mulch contact.
Plastic mulch does not allow water to pass to the soil below. While it might seem that this would be a problem, it can actually be beneficial when we have heavy rains or long periods of precipitation. Because of this you need to incorporate some kind of irrigation before you lay the plastic. While you could use something like a soaker hose, I prefer drip tape also known as T-tape because of its more consistent flow pattern. The drip tape will allow you to irrigate right at the roots of the plants which will help keep the walking rows drier and hopefully less weedy and help you conserve water. If you plan on planting two rows in the plastic you can have the drip tape in the center, if planting one row of plants you may want to lay the tape just off-center to avoid hitting the tape when planting or staking plants. A blow torch works well for making holes in the plastic to plant into. The plastic also helps keep the moisture from evaporating so quickly.
While the plastic does save time in weeding later in the season, it requires a greater investment of time up-front. Preparing the beds and laying the plastic is not an easy feat. It will take a minimum of two if not three people and a fairly calm day. You may be able to secure the edges of the plastic with heavy duty landscape staples, but to avoid problems with it being lifted up by wind I recommend you take the added time and effort to bury the edges making sure the plastic is as tight as possible to the soil surface. We were able to borrow a machine this year that mounds the soil, lays drip tape, pulls the plastic and theoretically buries the edges of the plastic. We soon found out that the conditions must be near- perfect for everything to go as planned – not too dry or the soil is too crumbly and rolls off the edges or not too wet or it can harm the soil structure and makes the soil cling to the disks rather than covering the edge of the plastic. Although we used the attachment, we still ended up having to go back and hand bury the majority of edges.
While I still see many benefits to using black plastic mulch, I will not use it for all crops and unless we grow in size considerably in the next year will take the time to lay it right by hand rather than try to use the layer.