Chapter 24

Greenhouse Pesticides

Herbicides

Herbicides are chemicals that inhibit or interrupt normal plant growth and development. Weeds of containerized crops in enclosed structures are often more problematic in nursery than greenhouse crops, primarily because most greenhouse crops are grown with higher quality media that is relatively free of weed seed. However, weeds still cause problems in crops often due to poor cultural practices. In greenhouses, weeds are a primary source of insects such as aphids, whiteflies, thrips, and other pests such as mites, slugs and diseases. Low growing weeds help maintain moist conditions, a favorable environment for fungus gnats and shore flies. Many common greenhouse weeds such as chickweed, oxalis, bittercress, jewelweed, dandelion, and ground ivy can become infected with Tospoviruses including impatiens necrotic spot virus (INSV) and tomato spotted wilt virus (TSWV) while showing few, if any visible symptoms. Thrips can then vector the virus to susceptible vegetable crops. Weeds can also carry other plant damaging viruses that are vectored by aphids. Few herbicides are labeled for use in a greenhouse due to the potential for severe crop injury or death to desirable plants. This injury may occur in a number of ways including: 1) spray drift if fans are operating at the time of application; and 2) volatilization (changing from a liquid to a gas). Herbicide vapors are then easily trapped within an enclosed greenhouse and injure desirable plant foliage. Always be sure the herbicide selected is labeled for use in the greenhouse.  Avoid use of preemergence herbicides in the greenhouse, which if applied to soil can prevent the emergence of seedlings. They can persist for many months and in some cases over a year. In addition, preemergence herbicides can continue to vaporize in the greenhouse, causing significant damage to young transplants. Several postemergence herbicides can be used under greenhouse benches and on the floors. If using herbicides outside, around a greenhouse, avoid using volatile herbicides that can easily enter the greenhouse ventilation system. Herbicides can be classified a number of different ways, which involve the following categories: 1) methods of application, 2) time of application, 3) mode of action 4) mobility, and 5) target site.

Methods of Application

Soil-Applied Herbicides

These herbicide groups have little or no foliar activity and are applied mostly for preemergence control of seedling grasses and some annual broadleaves.

Foliar-Applied Herbicides

Herbicides used on established weeds are called foliar-applied herbicides. They are separated into two groups, contact and systemic. Contact herbicides kill only the portion of green tissue that comes in contact with the herbicide.

Mode of Action

Herbicides can be grouped according to their mode of action (MoA). The mode of action is the way in which the herbicide kills weeds. It usually describes the biological process in the plant that the herbicide interrupts, affecting normal plant growth and development. Some of the different MoAs include cell membrane disruptors, growth regulators, photosynthesis inhibitors, pigment inhibitors, seedling growth inhibitors, and amino acid synthesis inhibitors. The Herbicide Resistance Action Committee (HRAC) developed a numbering system in which chemicals that have the same mode of action have the same HRAC number (See Appendix J, Herbicides Registered for Use in Vineyards). When selecting chemicals for rotation or mixing, select ones with different HRAC Group numbers because they have different modes of action.

Time of Application

Pre-Emergence

Pre-emergence herbicides are active in the soil against germinating weed seedlings and will not control weeds after they emerge. These herbicides are applied to bare soil and are leached into the soil with rain or irrigation where they kill germinating weed seeds. Best control results when activation, from rainfall or overhead irrigation, occurs within a few days of application. If herbicides remain on the soil surface without incorporation, some will degrade primarily by photodecomposition, hydrolysis, and microbiological activity—resulting in less weed control.

Post-Emergence

Post-emergence herbicides are applied directly on the weeds growing in the field. These herbicides are divided into two types: contact and systemic. Contact herbicides kill only those plant parts with which they make contact. Systemic herbicides are absorbed by roots, stems or leaves and are moved throughout the plant. Post-emergence herbicides can be combined with pre-emergence herbicides to control weeds that emerge later or applied as spot treatments during the growing season. Most post-emergence herbicide applied in vineyards are nonselective, or herbicides that control annual or perennial grasses, but not broadleaf weeds.

Mobility in the Plant

Contact Herbicides

Contact herbicides destroy only the plant tissue in contact with the chemical. Generally, these are the fastest acting herbicides. They are most effective on young and actively growing annual weeds but can be used to retard the growth of perennial weeds.

Systemic Herbicides

Systemic herbicides are translocated through the plant, either from foliar application down to the roots, or from soil application up through the roots to the leaves. They are capable of controlling perennial weeds and may be slower acting but ultimately more effective than contact herbicides.

Target Site

Selective Herbicides

Selective herbicides can be used to control certain plant species without injuring others. This characteristic can be used to control weeds while avoiding harm to desirable plants. Some selective herbicides may affect foliage of plants while leaving plant roots unaffected.

Non-selective Herbicides

Some herbicides are non-selective and must be used with extreme caution. They are used primarily in situations where complete removal of vegetation is desired, such as on transportation rights-of-ways.

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