Thursday, June 25, 2009

Got mulch?

With the arrival of temperatures in the eighties and nineties F along the Front Range, it’s time to mulch the vegetable garden. Soil temperatures are now thoroughly warm and mulches will not prevent the soil warming sought in spring.

The use of mulches now has several benefits. It will blunt the force of hard summer rains minimizing crusting and soil compaction. Mulch slows evaporation and can reduce irrigation needed by 25 to 50 percent. Stabilizing soil moisture to avoid wide swings from wet to dry promotes better quality vegetables and encourages the activity of beneficial soil organisms. Mulches applied now tend to keep soil temperatures cooler during summer and suppress weed seed germination.

Weed and seed-free straw (left) and grass clippings make some of the best mulches. Wood mulches are not recommended around vegetable plants, only for paths in the garden. Collect grass clippings from lawns that have not been treated with herbicides (“weed killers”) for at least a month.

Apply a thin layer of grass clippings (right) which will dry in five to seven days (below), then add the next layer. Two to three layers is generally enough to suppress weeds. Do not add thick layers which will mat, smell and limit water and air movement into the soil. Carefully apply around leafy vegetables as grass clippings are difficult to wash from the leaves and heads of greens.

Both straw and grass clippings can be tilled under in the fall and easily break down to enrich the soil. If straw isn’t desired for tilling under due to appearance, it can be raked off and placed in the compost bin.

Straw and grass clipping mulch photo credit: Carl Wilson


  1. Why is wood mulch not recommended for veggies?
    I love the idea of using grass clippings!

  2. We cultivate and dig in our vegetable garden soils so often that mulch inevitably gets mixed in and is no longer a mulch but a soil amendment. Think of succession planting in mid-summer for fall vegetable harvest, fall planting of cover crops for the winter, spring planting, etc.

    While this is not a problem with fast-decomposing grass or even straw, wood products require years to break down and take nitrogen from the soil while doing so. For annual crops like vegetables, it is better to keep wood chips and bark out of the growing beds. With perennial plants where digging in the soil occurs very rarely, the wood mulch stays on top and is of no concern from a soil fertility standpoint.

  3. Grass clippings aren't an option for us. Where can the average person get some weed-free straw?

    Beth, Lafayette

  4. What happens if there hasn't been adequate time between spraying a lawn and using it on veg plants? A friend brought over several bags that he said hadn't been sprayed for four weeks; however, my pepper and potato plant leaves are curling and look sickly. Do the chem compounds stay in the plants (if this is the cause of their appearance). Should I even eat them? Thanks much,

  5. The best source for answers to pesticide safety questions is the National Pesticide Information Hotline at 1-800-449-7587. You may have a sublethal dose on your plants and they will outgrow it. Tomatoes are particularly sensitive to 2,4-D type dandelion sprays and even vapor drift from neighbors can affect them if conditions are right.

  6. I have a straw bale that got wet with recent rains. Some mold is now growing on the inside. Is it still safe to use for mulch in my vegetable garden?

  7. The molds that grow on straw are similar to the microorganisms in soil that break down plant leaves. They don't affect growing plants, just recycle dead plant parts into soil nutrients for live plants to grow.

  8. How important is it for straw used for mulching or composting vegetable beds to be unsprayed? I'm having a hard time finding any. Is it okay to use if it was last spring that the straw was sprayed?

  9. With food gardening it is important to ask questions about what chemicals are placed in the soil. Not knowing what the treatment is makes it hard to say. What was the purpose of the treatment? Can you possibly ask the supplier what was sprayed?