Weed Control in the Landscape and Garden

FOR IMMEDIATE RELEASE
June 20, 2008

Weeds are a fact of life for the home gardener. But, understanding the types of weeds and how they grow can help us choose the best approach to manage them. Tony Bratsch, University of Illinois Extension horticulture educator, explains.

Weeds fall into two major categories, either broadleaf types such as dandelions or grassy weeds such as crabgrass. Either type can be an annual or a perennial. An annual grows, goes to seed and dies in one season. A perennial re-grows year to year. Perennial weeds usually sprout from an underground plant part such as a rhizome or a fleshy storage root. Another less common life cycle is the biennial weed, which grows in one season, over-winters and then flowers and goes to seed the following spring.

Weeds can be controlled culturally, which means without use of chemicals. Cultural methods include hand pulling and cultivating; flaming using a hand-held propane torch; and reducing seed in the soil by using a stale seed bed, avoiding introduction of weed seeds (in manure and hay), and never allowing weeds to go to seed. Using barriers such as fabric mulch and replacing soil with sterilized soil mixes in containers and raised beds can also be effective control methods.

Chemical control focuses on stopping weed seed germination by using a "pre-emergent" herbicide such as Preen^TM or by spraying existing weeds with a "post-emergent" material such as Roundup^TM.

Pre-emergent materials vary in their effectiveness against different weeds. Weeds controlled are listed on the product label. Timing the application before germination of seeds is critical. For example, crabgrass is an annual grassy weed which germinates in the spring when soil temperatures reach 60 degrees and above. Thus, timely spring application is needed for crabgrass control products to be effective. If they are applied too soon, the material may lose its effectiveness before the weeds germinate. Winter annuals such as henbit and chickweed in turf germinate in the mid-fall as soils cool. These weeds grow slowly all winter and go to seed in the spring. A well-timed early fall application using a pre-emergent herbicide can prevent these weeds from becoming established.

Post-emergent herbicides can be contact kill only or translocated into the plant's system, moving downward to kill underground root parts. Post-emergent materials can also be selective or non-selective in the types of weeds controlled. Selective materials usually either kill only grass or only broadleaf weeds -- but not both.

So what is the best approach? Bratsch says the answer depends on the weed and situation, but can involve either or both cultural and chemical methods.

For lawns, both grassy and broadleaf weeds can be a problem. In addition to using timely pre-emergent materials in the spring and fall, selective post-emergent broadleaf materials such as 2,4-D and dicamba can be applied to turf without injury to grass. These materials are often sold with fertilizers as a dry product that sticks to leaves, but they can also be sprayed on as a liquid formulation.

In landscape and perennial beds, mulching, hand-pulling and cultivating can be effective. Fabric mulches are useful; but, over time, weed seeds can blow in and start growing in the mulch on top of fabrics. In new beds, an application of Preen^TM can help. Spot spraying with a post-emergent product such as Roundup^TM must be done with care in landscape beds because even a light breeze can blow the spray material onto desirable plants and damage them. Vegetable gardens planted in rows can be mechanically cultivated or hoed by hand. Always remember that a young weed is more easily killed than an older weed. Preen^TM is also labeled for use in gardens but should only be used around transplanted vegetables and perennial crops such as asparagus and rhubarb. Keep in mind, vegetable and flower seeds sown directly in the garden can be affected by this herbicide.

Bratsch says perennial weeds in all gardening situations present a special challenge. Quackgrass, Johnsongrass, nutsedge, field bindweed, pokeberry, poison ivy and Japanese honeysuckle are examples of persistent perennial grass and broadleaf weeds. Often, hand pulling and cultivating only spreads them further and only removes the top growth. Repeated removal of top growth over the season can help deplete root reserves and weaken the weeds, but this can take several years before control is realized. Translocated herbicides such as Roundup^TM and 2,4-D are good tools to manage perennial weeds, but the applications should be well-timed and the appropriate materials matched with the weed being controlled. Good timing involves spraying after the plants have begun growing and are actively sending storage nutrients to stems and underground plant parts for next year. So, a mid- to late summer application is ideal. Round-up^TM is a better grass than broadleaf herbicide; but, at high rates it will kill many broadleaf weeds. 2,4-D is only effective on broadleaf weeds.

With proper approaches and tools, weeds can be managed to maximize the beauty of your landscape and the productivity of your garden. For more information on weed and pest control, visit the U of I Extension website, www.extension.uiuc.edu.

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