Mulch can be one way to help plants remain healthy and growing while water is scarce and temperatures climb into the mid-80's and higher. Mulches, and in particular lighter-colored organic mulches, shade the soil to keep it cooler, which in turn helps to reduce water loss from the soil and the plants growing there. As plants warm up, they use the process of transpiration to keep from getting too hot. Transpiration is sort of like perspiration in that water vapor exits the leaves through tiny pores called stomata. As that water vapor evaporates, it cools the leaves. It is very important for plants to not get too hot. Since a mulched soil will not lose as much water as a bare soil, it will be able to provide a source of water to the plants for a longer period of time. If plants run out of water that they can absorb from the soil, they will close up their stomata to prevent additional water loss, but this then can cause the plants to overheat. The stomata are also used to allow carbon dioxide to enter the leaf, where it is used for photosynthesis. Oxygen produced from photosynthesis also exits the leaf through the stomata. Therefore, If the stomata are closed, this can slow or even shut down photosynthesis. Furthermore, if the plant continues to heat up and wilt significantly, it can die.
There are many different materials that can be used as a mulch around vegetables and flowers. My preference is to use an organic mulch whenever possible. One of the primary reasons for this is that these kinds of mulches will break down over time to add nutrients and organic matter to the soil. Inorganic mulches, like plastics, can work very well and may prevent weeds better than some organic mulches, but they will generally need to be removed at some point and disposed of.
Probably the most common organic mulch that people use is grass clippings. These can work very well in the vegetable garden. However, they should be applied carefully so that you do not apply a layer more than1/2” deep at one time. Multiple applications can be made but they should not ever get more than 3" to 4” deep. They shade the soil, conserve moisture, and can reduce weed problems. However, if they mat down, they can impede water absorption into the soil, and during wet conditions they can give off a bad odor too. You can decrease weed seed germination by first applying a few layers of regular newspaper to the ground, then cover that with the grass clippings. Water it all down to help the clippings to adhere to the newspaper and help it to stick to the soil so it does not blow away as easily.
But one has to be careful as to the source of those clippings and the history of the grass that produced them. Lawn herbicides can still be present on grass clippings, even weeks after they were applied. Sensitive plants like tomatoes and potatoes can show damage from herbicides like 2,4-D and Banvel within just a few days of using contaminated clippings around plants. Generally it will take 1 to 2 months for 2,4-D to break down to a safe level, once it has been applied to a lawn. Banvel can take 3 to 12 months! Some of the other commercial products will need a similar time frame to effectively break down. So, while these are commonly used, probably the best place for your grass clippings is on the lawn! As long as you mow on a regular basis, not removing more than 1/3 of the grass blade height at one mowing, a regular lawn mower should distribute them nicely over the surface of the lawn where they will filter down in between the grass plants and decompose, recycling that expensive fertilizer you may have applied to your lawn as well as adding valuable organic matter to the soil.
Leaves and Pine Needles
Leaves and pine needles are another organic mulching material to consider. However, most people do not have a lot of leaves in the early summer to use as a mulch for their gardens. The answer is to plan ahead by bagging your dry leaves in the fall, store them for the winter and spring, then use them as a mulch the next year.
Straw and Hay
Straw or hay are other organic options. Of the two, the hay may be the better choice, particularly if it contains alfalfa or clover, both of which will provide a higher amount of nitrogen to the soil as they decompose. Once again, you need to be careful of potential herbicide contamination in the straw and both might be contaminated with weed seeds. Look for certified weed-free straw which should be free of any weed seeds. It might still have some grain seed in it, but this is usually of minimal concern and can be pulled easily. Straw works particularly well as a mulch around potatoes, where you would ordinarily be hilling the potatoes to encourage higher production.
Vegetables and Greens
Living mulches are gaining interest. In this case you actually plant a variety of vegetables, often in a raised bed, close enough together that there simply isn’t room for weeds to grow. Each plant helps to shade the soil, keep it cool and reduce weed seed germination and growth. For example, you might plant lettuce or spinach around your broccoli transplants. As the salad greens grow, they can be harvested and the broccoli will grow nicely above the top of the lettuce and spinach.
Some people are using leguminous plants like perennial Dutch white clover. You can plant it in the aisles between the rows of your vegetables. It helps compete with weeds, gives you a nice, soft path to walk on and works particularly well with larger plants that are grown in the rows. Clovers can also fix nitrogen which can become available to vegetable plants.
Some people like to let purslane grow in between their vegetables and some even say it makes their tomatoes taste better. They also like to harvest the purslane and add it to their salads. But many gardeners consider purslane one of their most undesirable weeds and do their best to keep it out of their garden.
Cover cops like oats, buckwheat, rye, or legumes can also be incorporated into the garden. These are usually planted during a fallow year for the garden, in the fall after the garden is finished or early in the spring before the regular garden is planted. The vegetation is then usually mowed or rolled down to cover the soil or tilled into the soil. Cover crops can provide erosion protection as well as a great source of organic matter for your garden’s soil.
Paper and Cardboard
Shredded office paper can be used, but like the newspaper mentioned earlier, it will blow away in the wind if it is not wetted down right away and then flattened into the soil. Cardboard is another option but it too can blow away and it may impede water absorption into the soil, but that may not be a real problem in between rows of plants. It is effective in preventing weed growth however and can be held down by using landscape pins.
Inorganic mulches are fairly popular as well. These are usually plastic mulches or films that are laid down on top of the soil or over a shaped bed. Often drip irrigation tapes or lines are laid beneath the plastic to facilitate irrigation. Vegetables are transplanted into holes that are punched or cut into the plastic. Weed barrier fabrics may also be used, but are usually much more expensive. Home gardeners may find these mulches very appealing since you can also use an ooze hose for irrigation beneath the plastic or fabric. Market and commercial growers often use inorganic mulches to reduce weed management and other costs. Since these mulches are usually black, they help to warm the soil early in the season, giving warm-season vegetables a quicker start than if they were planted directly in the soil. Inorganic mulches may actually keep the soil cooler early in the season. However, picking up and disposing of the plastic can be a real chore and environmental expense.