By: Sandy Feather ©2006
Penn State Extension
Q. I plan to use straw as a mulch over my vegetable garden for the winter. Is it OK to put mulched leaves on the garden before covering it with straw?
A. Leaves that have been composted or shredded make a good winter mulch for the vegetable garden. Whole leaves can mat down and actually repel water, so it is important they be shredded first. You do not need a fancy shredder; just run them over with your lawn mower. You can either use a bagger, or mow them into windrows. The only other caution is to make sure that none of the leaves are from a black walnut or butternut tree.
These trees produce a compound, juglone, that hinders the growth of many plants near those trees. Juglone is concentrated in the roots, buds and nut hulls but is also found in the leaves and stems. Tomatoes are extremely sensitive to juglone.
Q. A weed called purslane takes over my vegetable garden every year. Can you suggest ways to control it?
A. Purslane is a summer annual weed that thrives during hot, dry weather. It is a low-growing, succulent plant that often forms a dense mat over the soil surface. Like most annual weeds, it is prolific seed producer.
The best approach to controlling it is to use several different control options in combination. Begin hand weeding and shallow cultivation as soon as you notice purslane seedlings germinating. Mulching is another effective control. A 3-inch layer of an organic mulch such as shredded leaves, straw or composted, herbicide-free grass clippings will inhibit the germination of purslane seeds.
Mulch should be applied after the garden has been planted and the vegetable plants are up and growing. It is helpful if weeds have been eliminated before applying the mulch.
Preen (trifluralin) and other pre-emergent herbicides -- those that work by preventing successful weed seed germination -- can reduce the amount of purslane in your vegetable garden. Preen will not control existing weeds, so it is important to eliminate standing weeds before application. Follow label directions carefully to avoid injury to the crops you are growing.
Since weeds germinate through the growing season, Preen can be reapplied every eight to 12 weeks. It should be lightly tilled or watered into the top inch or two of soil to activate its weed control properties and to protect it from breaking down on exposure to sunlight.
Organic gardeners may use corn gluten meal. This is the active ingredient in the Gardens Alive! WOW product as well as Preen Vegetable Garden Weed Preventer. It is also a pre-emergent herbicide and should be used much like regular Preen. But it must be reapplied every four to six weeks to maintain control.
Avoid using pre-emergent herbicides in areas of the garden where you want to direct-seed crops such as lettuce, corn or root crops.
Q. Can I use sawdust as mulch after my garden has been planted?
A. Sawdust can be used as mulch and is often recommended around blueberries and other plants that prefer an acid soil. It is best if the sawdust has been composted for at least six months (preferably one year).
If not, it can tie up available nitrogen in the soil and create a deficiency in the plants growing there. You can also add 1/2 pound of actual nitrogen per cubic yard of sawdust to avoid the problem. Very fine sawdust can pack down and interfere with the movement of water and air through the soil. Be sure to stir it up periodically to avoid this problem.
A 2- or 3-inch layer is sufficient to help keep weeds down, maintain soil moisture and moderate soil temperatures. Avoid piling it up around the stems of plants, especially if it has not been composted thoroughly.
To determine how much of a given fertilizer is necessary to apply 1/2 pound of actual nitrogen, divide 0.5 by the percentage of nitrogen in the fertilizer.
For example, 10-10-10 is 10 percent nitrogen, 10 percent phosphorus and 10 percent potash, always in that order. 0.5 divided by 0.1 equals 5 pounds of 10-10-10 fertilizer added to a cubic yard of sawdust. It is helpful to choose a fertilizer that contains only nitrogen, such as urea (46-0-0). It will take about 1 pound of urea to add one-half pound of nitrogen.