Herbicides are chemicals that are utilized to kill weeds in lawns and gardens. The scores of herbicides that exist attack weeds in different ways. Herbicides fall into two general classes, however. Some are selective herbicides formulated to kill specific weed species while leaving the rest of the plants unharmed. Other herbicides are non-selective, formulated to kill all plant life wherever they’re applied.
Methods of Destruction
1 general class of herbicides strikes weeds’ or all plants’ cell structures. Some products in this category mimic plants’ natural growth hormones to trigger uncontrolled cell growth that breaks down plant constructions while other products destroy enzymes the plants need to build and maintain cell walls. Another general category of herbicides disrupts the chemical process of photosynthesis, by which plants convert sunlight energy to food energy. A third general category of herbicides disrupts plants’ ability to synthesize the amino acids required to soften food energy for growth and reproduction.
Before or After Germination
Herbicides are also classified by if they behave before or after plant seeds germinate. Pre-emergent herbicides usually are put on the ground. They soak in the soil and prevent seeds from invading or kill the sprouts as soon as they break the seed coat and before they get to the ground surface. Post-emergence herbicides are applied to weeds or other plants that already sprouted. Post-emergence herbicides are split into contact and systemic products. Contact herbicides kill the plant parts they touch, like the leaves, but aren’t transported to the roots. They work well against annual weeds. Systemic herbicides travel throughout plants to kill roots and the rest of the plant parts, and they’re most effective against perennial weeds.
Age and Weather
Weeds’ age and the weather affect herbicide activity. Young, actively growing weeds are more susceptible than fully mature weeds to herbicides, and a few grass species develop herbicide-repelling leaf hairs or waxy leaf coatings as they mature. Rainfall close on the heels of the herbicide application can wash off the weedkiller before it can act. Really dry weather conditions may cause fluid herbicides to disappear before they penetrate leaves. Dry conditions also result in leaf pores to close, blocking their uptake or absorption of the herbicide. Herbicides can be affected by light, also; a few break down quickly in bright light so should be applied just in night hours. Low temperatures and high winds also tend to decrease herbicide uptake.
Herbicide Resistance
Over the years, weeds can become immune to specific herbicide products through natural selection. Individual bud plants that survive herbicide applications grow and spread their seeds, creating future generations of more-resistant weeds. Eventually, all the weeds are immune. Strategies for preventing or minimizing herbicide resistance include crop rotation, using herbicides with various modes of activity singly or in mixtures, killing weeds in fallow areas to prevent the spread of resistant weeds and combining mechanical techniques like pulling weeds with herbicide procedures for weed management.