There are health and safety risks involved in chemical use, which you need to consider. You also need to think about weather conditions and when you should start your spraying. It’s important to get timing right when using agrichemicals. Herbicides need to be applied during the active growing season of the weed species. This is usually spring and autumn.
Some herbicides, such as glyphosate, kill weeds on application and leave no residue in the soil. Others, called residual herbicides, can continue to be effective for some time after application because they persist in the soil. You must take care with residual herbicides. Due to their longevity in soil, they can have an impact on seedlings that are subsequently planted.
There are 3 main ways to apply herbicides:
If pest trees, such as sycamore, are on your site, you can treat them with herbicide through techniques such as drill and fill, ring barking, or basal spraying.
Different species of pest plants need different chemicals to treat them, as well as different methods for removal. Regional councils generally provide good information on treating pest plants. For information about weed species and treatments to use, see the Weedbusters website.
If your planted area is small, 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 can spot spray and plant the next day, while others will need time for the vegetation to die off and be visible. A residual herbicide may remain in the soil for some time, so you’ll need to delay planting. Ensure that you take note of the withholding period (the period between spraying and planting) for any chemical that’s used. Be careful to ensure correct spot size is maintained as more chemical on a smaller spot will have a higher concentration.
Blanket spraying is effective and time efficient for higher density planting and larger planting areas. On some planting sites, it can be done using a knapsack sprayer. On very large sites, 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 or cutover forestry land.
If you're undertaking blanket spraying at a larger scale, keep an eye on your soil and monitor for other environmental impacts. Your site may suffer soil or surface erosion and dried out ground, which could result in 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. Be aware that when treating land at scale, you may also impact existing vegetation already on site. It is important to avoid spraying desirable existing vegetation, such as native bush.
Identify the different land uses on neighbouring property. For example, if there are adjacent horticultural properties, aerial spraying of herbicides will need extra planning and supervision.
For scattered pest plants, cutting and pasting stems of vines and weeds with herbicide is a good option. This limits the amount of chemical that is used. As with any herbicide, familiarise yourself with the effects of the chemical being applied and its potential to translocate (move from one plant to another) into surrounding, desirable vegetation.
When cutting and pasting, it’s essential that the herbicide is applied to the cut stem within 30 seconds of it being cut. This ensures the poison travels through the plant while the sap is still running. Some plants may need more than one treatment if they sprout from cut stems. Different pest plants will need different products.
One of the biggest considerations when using chemicals is time. If you do not get the timing right, you can lose money as your treatment will be less effective. This can vary depending on the method used, but in most instances, herbicides should be applied during the growing season. Familiarise yourself with the plant being treated, the chemical being used, and the method you intend to use.
Other than time, there are several things you need to think about, including:
Different regional councils have different rules about notification when spraying large areas. In some instances, you may need to notify your neighbours, while in other regions this is only needed when using motorised spray.
If you're treating large areas, talk to your regional council land management officer as the use of chemicals is regulated by councils and the Resource Management Act 1991. They can 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.
For further information, contact your regional council, Farm Forestry New Zealand, or the Forest Owners Association.
Do not use any chemicals without learning about the impacts on both human and plant health. There are factors that can cause unwanted plant loss if not understood. These include:
If you’re planting native species, you need to take extra care as they're generally more susceptible to chemical effects. To protect the seedling, use plant guards or a spray guard when applying herbicides. Plant guards will also protect your seedlings from browsing by rabbits and hares – and reduce desiccation (drying out) from wind. If your planting area is small, you may prefer to use hand tools rather than chemicals.
Tāne's Tree Trust has information on using herbicides when preparing grass sites for planting natives.
There’s a lot to consider when you use chemicals to clear your land. Make sure you:
You may wish to do a course on the safe handling of chemicals, for example through Growsafe. Growsafe also has information on safe, responsible and effective use of agrichemicals.
WorkSafe has a section on managing hazardous substances safely.
Within 2 to 3 months of planting, it is likely you'll need to do some releasing (applying herbicides or hand weeding).
For hand weeding, you can use tools to remove long grass or choking weeds, but it’s labour intensive. For applying herbicides, this could be spot spraying or blanket spraying over the entire site.
If you blanket spray, you should be cautious of spraying seedlings. There are specific chemicals that will affect certain types of plants but have little or no impact on others. For instance, there are products that kill grass plants, but not shrubby plants. Again, understanding what a chemical is used for, how to use it, and what its impact will be is critical.
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.