Weeds compete with the crop for water, nutrients and light, shelter insects and diseases, make irrigation and harvest harder, and cost more the longer they are left. But the wrong herbicide can burn the crop itself or do nothing to the target weed, so the choice starts with knowing the weed, its stage and where you are spraying.
This guide explains the types of herbicides and how they work, the difference between selective and non-selective, and between pre-emergence and post-emergence products, and how to choose the right one and use it safely.
Main types: selective and non-selective, systemic and contact, pre-emergence and post-emergence.
First question before buying: what is the weed? Grass, broadleaf or a perennial such as nutsedge? And where does it grow: in a standing crop, bare land, or paths and uncultivated areas?
Common mistake: spraying a non-selective herbicide near the crop or on a windy day. Drift burns neighbouring plants.
Rule: read the label, use the right sprayer, wear protective equipment, watch the wind and respect the safety interval.
What are herbicides?
They are chemicals that kill weeds or stop their growth. Unlike insecticides and fungicides, they target plants, so great care is needed to keep them off the crop and neighbouring gardens. They are classified in more than one way, and combining these classes is what identifies the right product.
First: selective and non-selective
- Selective: kills a group of weeds without harming the crop named on the label, such as grass herbicides in broadleaf crops, or broadleaf herbicides in grass crops.
- Non-selective: kills most green plants it touches, so it is used on bare land, before planting, around fences and paths, or between rows with great care and protective shields.
Second: systemic and contact
- Systemic: the plant absorbs it and it moves inside down to the roots, so it kills perennial weeds and takes days to show its effect. Glyphosate is an example.
- Contact: burns only the green parts it touches. The effect is fast, within hours, but it usually does not kill the roots, so perennial weeds may return. Diquat is an example.
Third: pre-emergence and post-emergence
- Pre-emergence: sprayed on the soil before weed seeds germinate, or right after planting, forming a layer that stops germination. It needs level, moist soil and usually water or light rain to spread through the soil.
- Post-emergence: sprayed on weeds that have already emerged, while they are still young, because they are easier to control and less tolerant.
Fourth: by weed type
- Grass weeds (narrow-leaved): leaves are narrow and long, such as annual grasses, and there are dedicated herbicides that do not harm most broadleaf crops.
- Broadleaf weeds: broad leaves with net-like veins, such as amaranth types, and there are dedicated herbicides for them.
- Perennial weeds and nutsedge: they have roots and tubers that regrow, so they need a systemic herbicide or a product specialised against nutsedge, plus follow-up.
How to choose the right herbicide
- Identify the weed: grass, broadleaf or perennial, and its stage.
- Identify the location: standing crop, land before planting, or bare land and paths.
- Choose the type: selective for the crop field, non-selective for uncultivated land.
- Check the label: is your crop listed, and what growth stage does it allow?
- Mind the timing: pre-emergence or post-emergence.
- Plan rotation of active ingredients: to reduce resistant weeds.
Safe use
- Read the whole label: permitted crops, safety interval, warnings and any limits on irrigation or rain.
- Keep a dedicated herbicide sprayer: and use another pesticide sprayer only after washing it thoroughly, because small residues can burn plants.
- Watch the wind: spray on a calm day, with coarse droplets and a low boom to reduce drift.
- Avoid spraying right before rain or in extreme heat.
- Wear protective equipment: gloves, goggles, a mask and long sleeves.
- Wash the equipment well after use and dispose of the rinse water away from plants and drains.
- Do not mix without knowing: some products clash or harm the crop.
For more on spraying equipment, read Types of Agricultural Sprayers and The Electric Agricultural Sprayer.
Common mistakes
- Spraying a non-selective herbicide beside the crop or on its leaves.
- Using the same sprayer for other pesticides without washing.
- Spraying on a windy day so droplets drift to neighbouring fields.
- Waiting until weeds grow large and flower before controlling them.
- Relying on one product from the same group season after season.
- Applying pre-emergence products to very dry or rough soil.
- Ignoring the safety interval.
Weed resistance to herbicides
Repeating the same active ingredient or mode of action gradually produces resistant weeds. To limit it, rotate between herbicide groups, combine chemical and mechanical control, use pre-emergence together with post-emergence products where needed, and do not let surviving weeds set seed.
Control methods that complete the herbicide
- Organic or plastic mulch: it blocks sunlight and reduces germination.
- Mechanical hoeing: between rows before weeds grow.
- Cleaning equipment: so it does not carry seeds from one field to another.
- Targeted regular irrigation: without wetting empty areas.
- Good fertilization: it strengthens the crop so it competes with weeds.
Products from the Orkida Store
We start by identifying the weed and the location, then choose the right herbicide. We advise following the label dose and the safety interval.
- Touchdown System 4 360 SL Glyphosate Herbicide 5 L – Syngenta: from Syngenta; a systemic non-selective glyphosate herbicide for annual and perennial weeds on uncultivated land and before planting.
- Reglone 200 SL Diquat Non-Selective Herbicide by Syngenta 5 L: from Syngenta; a fast-acting contact diquat herbicide that burns the green parts of grass and broadleaf weeds.
- Fusilade Max 125 EC Grass Herbicide 1 L – Fluazifop-P-butyl: from Syngenta; a selective fluazifop herbicide for post-emergence control of grass weeds in broadleaf crops.
- Laser 10% EC Cycloxydim Grass Herbicide by BASF 5 L: from BASF; a selective cycloxydim herbicide for post-emergence control of grass weeds.
- Authority 480 SC Sulfentrazone 48% Herbicide 1 L – FMC: from FMC; a sulfentrazone herbicide for broadleaf weeds, nutsedge and amaranth types.
- Sakura 850 WG Pyroxasulfone 85% Herbicide by Kumiai 150 g: from Kumiai; a selective pyroxasulfone herbicide for annual grass weeds.
- Glypho 48% SL Glyphosate Non-Selective Herbicide 5 L: a systemic non-selective glyphosate herbicide for bare land, paths and field edges.
- Pendalin Plus 330 EC Pre-Emergence Herbicide 1 L – Pendimethalin: a selective pendimethalin herbicide applied before emergence to stop grass and broadleaf weeds from appearing.
- Bragger 28% EC Bromoxynil Broadleaf Herbicide 1 L: a selective bromoxynil herbicide for broadleaf weeds after emergence.
Frequently asked questions
What is the difference between selective and non-selective herbicides?
A selective herbicide kills a specific type of weed without harming the crop named on the label, while a non-selective one kills most green plants it reaches.
When does a herbicide show its effect?
A contact product within hours; a systemic one such as glyphosate within days before the weed dies completely.
Should I spray before or after emergence?
Pre-emergence stops weeds appearing; post-emergence kills young weeds that are already there. The product and the crop set the timing.
Can I use my insecticide sprayer for herbicides?
It is best to keep a dedicated herbicide sprayer. If you have to share one, wash it thoroughly, because residues can harm the crop.
Why did the weed not die after spraying?
It may have been too big, the product may not suit its type, rain may have washed it off, or the weed may be resistant to the active ingredient.
Does the herbicide harm the soil or the next crop?
Some products leave residues in the soil that affect a following crop, so check the label before changing your rotation.
If you are not sure which weed you have, or which product suits your crop, visit your nearest Orkida branch with a photo or a sample and we will point you to the right product.