Using herbicides is a common practice to control competing vegetation before and after planting. What you use will depend on your planted area, vegetation cover, what you're planting and how the chemicals might affect your seedlings. You also need to think about weather conditions and when you should start your spraying.
What types of chemical spraying are there?
Some herbicides, such as glyphosate, kill weeds on application and leave no residue in the soil. Others, called residue herbicides, can continue to be effective for some time after application because they persist in the soil.
There are 2 main ways to apply herbicides – individual spots or blanket spraying the whole site (aerial dessication spraying).
If your planted area is smaller in size, you might want to do pre-plant spot spraying. This allows you to spot spray at the intended planting density. It also uses less chemical and reduces the risk of spray drift.
With some chemicals, you may be able to spot spray and plant the next day, while others will require time for the vegetation to die off and be visible. Be careful to ensure correct spot size is maintained as more chemical on a smaller spot will have a higer concentration.
Blanket spraying (aerial dessication spraying)
Aerial spraying is effective and time efficient for higher density planting and larger planting areas. It can be done by helicopter or machine on flat terrain. It’s a useful form of spraying to knock down vegetation quickly on unproductive pastoral land. Keep an eye on your soil and monitor for other environmental impacts – you may suffer soil or surface erosion, dried out ground and reduced survivability.
You should use professionals for this form of spraying who will provide advice on the correct chemical composition for your land. For example, woody and weedy cover will need different chemicals to gorse or herbaceous cover.
Identify the different land uses on neighbouring property. For example, if there are adjacent horticultural properties, aerial spraying of herbicides will require extra planning and supervision.
Talk to your council, Farm Forestry New Zealand, or the Forest Owners Association to find an agency in your region.
When is the best time to spray?
One of the biggest considerations when using chemicals is time. If you do not get the timing right, you can lose money as your spraying may be less effective.
You can spray at any time, but autumn pre-plant spraying when vegetation is actively growing will often give you the best result. At other times of the year, you may need to use a higher chemical rate to get the same effect. Some weeds are managed more easily if sprayed at a certain growth stage. However, if sprayed too early they may regrow, meaning a second spraying could be necessary.
What other factors can affect when I spray?
Other than time, there are several things you need to think about, including:
- weather conditions as wind can cause your spray to drift, and heavy rain can cause soil loss and degradation and wash away chemicals
- getting grass low for less chemical usage and easier application
- having enough time before planting for chemicals to break down and kill vegetation
- allowing a longer period if you pre-spot spray so you can see where to plant
- allowing time for more than 1 spray (you may need to spray the year before you plant), for example, control of heavier, woody vegetation
- the best type of spraying for your land.
Who should I talk to about chemical spraying?
Before you begin, you should talk to your regional council land management officer as the use of chemicals is regulated by councils and the Resource Management Act 1991. They’ll be able to help you find someone to talk to about chemical usage and the best approach for your land.
Agrichemical providers and contractors are also a useful source of advice.
Will chemicals affect my seedlings?
Some tree species are more susceptible to chemicals than others. You need to make sure what you use is compatible with what you plant, for example, eucalypts and many other hardwood species are sensitive to herbicide sprays, so when planting in pasture a pre-plant spot spray is often a better option than post-plant releasing. Lighter chemical mixes may not give the same knock down rate, but do allow your seedlings to carry on growing.
Planting native species
If you’re planting native species, you might need to take extra care as they're generally more susceptible to chemical effects. To protect the seedling you might do less in the way of pre-plant spraying and use other methods of vegetation control instead. Pioneering species like open ground, while other species may benefit from vegetative competition around them.
Tane's Tree Trust has information on using herbicides when preparing grass sites for planting natives.
Health and safety when using chemicals
There’s a lot to consider when you use chemicals to clear your land. Make sure you:
- follow product labels and safety data sheets
- follow manufacturers recommendations
- use protective clothing
- use the most appropriate equipment
- avoid drift spray to neighbouring properties and waterways
- spray in calm conditions and when no rain is expected
- prevent species from being sprayed that are not meant to be.
Growsafe has information on safe, responsible and effective use of agrichemicals.
WorkSafe has a section on managing hazardous substances safely.
Post planting maintenance after spraying
Within 2 to 3 months of planting it is likely you'll need to do some establishment maintenance (releasing). This could be spot spraying or blanket spraying (over the entire site). If you blanket spray you should be cautious of spraying seedlings.
You'll need to prevent other vegetation from competing with your seedlings for at least 3 months to 1 year after planting for exotic species, and up to 4 years for native seedlings.