In Minnesota, it's as important to mulch garden and landscape plants in late spring as it is in late fall. Mulch insulates the soil and protects plant roots from extremes -- extreme heat in summer, extreme cold in winter and premature warming in spring. But summer mulch does much more than help plants beat the heat.
Weed and feed
Windy weather and hot sunlight beat down on exposed, dark soil and can strip away moisture in a hurry. Summer mulch conserves moisture by reducing the amount of surface evaporation. It also helps prevent erosion caused by strong winds.
Many gardeners know that mulch helps in the eternal battle against weeds. Summer mulch prevents light from hitting the soil, which keeps the soil cooler and reduces the number of weed seeds that are able to germinate and compete with desirable plants for food and water. The weeds that do manage to sprout under a layer of summer mulch usually yield to a gentle tug.
But few gardeners are aware that summer mulch also serves to cushion garden soil from the cumulative blows from rainfall and sprinkler water. It doesn't seem as though tiny droplets of water could have a negative impact on soil, but over the summer rain pounds on the soil, packing it down and often leaving it crusty as it dries. Beneath a layer of mulch, however, soil remains softer, looser and more friable, which provides a better environment for plant growth.
In addition, organic mulches such as grass clippings, straw, chipped leaves and bark break down, eventually adding small amounts of nutrients to the soil. One caution: As mulch breaks down, it uses nitrogen. Monitor your plants to make sure their foliage is a healthy green and that they're growing at a reasonable rate. If not, provide additional fertilizer.
Chips and coffee
Turning wood chips or shredded bark mulch into the soil at the end of the season is practically guaranteed to create a nitrogen deficiency the following year, so be sure to add extra fertilizer any time you work them into the soil. A better idea would be to reserve the use of woody mulches for areas that you don't work often, such as around young trees and shrubs or in perennial or bulb gardens.
Some coffee shops give away large bags of coffee grounds for use as garden mulch. While the price is right and coffee ground mulch is attractive, its dark color absorbs heat, which reduces its insulation value. Because coffee grounds can cake, you should apply no more than an inch over the soil surface and renew it as needed. Another option would be to add those coffee grounds to your compost pile, then use partially completed compost for mulching the garden.
Leaves and grass
Leaves aren't usually plentiful in spring, so if you'd like to use them for summer mulch, stockpile them in the fall. Save plenty of dry leaves in large plastic bags. It's a good idea to run your lawnmower over them several times to chop them up before bagging them. Come spring, you can spread a thick layer of chopped leaves on your garden.
Oak leaves are particularly valuable because they're slower to break down and they acidify our alkaline soils slightly, which is good for the vast majority of plants we grow here.
Unlike leaves, grass clippings are plentiful now, but unless you apply only a thin layer at any one time, it's better to dry them first. Fresh grass clippings may be smelly, and a thick layer can produce a fair amount of heat as it decomposes. If you've used any sort of herbicide in the lawn, be sure to wait until you've mowed at least two or three times before collecting the clippings for mulch.
Pine needles, like oak leaves, will acidify the soil slightly. All you need is about an inch or so to give you good results. Pine needles look good, insulate well and are relatively slow to break down, so you needn't keep adding to them as the summer progresses. Pine needles can also be spread around acid-loving woody plants, such as azaleas, rhododendrons, blueberries and the new blue-flowering hydrangeas.
Deb Brown is a horticulturist with the University of Minnesota Extension Service Yard and Garden Line. For help with garden, plant and insect questions, call the Yard and Garden Line at 612-624-4771.