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 more effectively.
Weeds fall into two major categories, either broadleaf types such as dandelions, or grassy weeds such as crabgrass. Either type can have an annual life cycle, which means it grows, goes to seed and dies in one season, or it is a perennial, which means it 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 without use of chemicals. These methods include hand pulling and cultivating, flaming using a hand-held propane torch and reducing seed in the soil by using the stale seed bed technique, 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 of weeds focuses on stopping weed seed germination by using a "pre-emergence" herbicide such as PreenTM , or by spraying existing weeds with a "post-emergence" material such as RoundupTM.
Pre-emerge 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 60F and above. Thus timely spring application is needed for crabgrass control products to be effective. If they are applied too soon, the material may loose 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-emerge herbicide can prevent these weeds from becoming established.
Post-emergence herbicides can be contact kill only, while others are translocated into the plant's system, moving downward to kill underground root parts. Post-emergence materials can also be selective or non-selective in the types of weeds controlled. Selective materials usually either kill grass or broadleaf weeds.
So what is the best approach? This 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 well timed pre-emerge materials in spring and fall, selective post-emerge 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, or they can be sprayed on as a liquid formulation.
In landscape and perennial beds, mulching, hand-pulling and cultivating can be very effective. Fabric mulches are useful, though over time, weed seeds can blow in and start growing in the mulch on top of fabrics. In new beds, an application of PreenTM can be a valuable tool. Spot spraying with a post-emerge product such as Round-upTM should be done with care in landscape beds as a 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 hand-hoed. Always remember a young weed is more easily killed than an older weed. PreenTM 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.
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 serves to spread 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 RoundupTM 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. Thus a mid to late summer application is ideal. Round-upTM is a better grass than broadleaf herbicide, but at high rates will kill many broadleaf weeds. 2,4-D is only effective on broadleaf weeds.
Weeds do not have to be an overwhelming aspect of a gardener,s life, and with proper approaches and tools, they can be easily managed to maximize the beauty of your landscape and the productivity of your garden. For more information on weed and pest control visit our University of Illinois Extension website: http://web.extension.uiuc.edu/state/hort.html
Source: Anthony Bratsch, Extension Educator, Horticulture (Serving East-Central and Southeastern Illinois), firstname.lastname@example.org
Pull date: September 1, 2008