Site Preparation for Sowing

Site Preparation for Sowing Native Grass Seed

The ideal site to sow native grass seed is one where the soil is free of weeds and weed seed. If you want to sow other herbaceous species like wildflowers with your grass seed mix it is essential that the soil is also reasonably low in nutrients. If the soil you use is high in nutrients you will find that grasses will form a full cover and out-compete most forb species that have also been sown.

If the site you plan to sow with native grass seed is very weedy there are usually three options to reduce the weed seed problem.

Spray the site repeatedly over a peoirod determined by the amount and/or species of weeds.
Scrape a layer of topsoil containing weed seed from the site.
Import weed free soil.

These methods have their pros and cons.

Spraying, or Spraying and Cultivating

Spraying a site will probably never remove all the weed seed, and the period of time for spraying different sites is difficult to determine. A combination of spraying and cultivating the soil will usually result in a much higher reduction of the soil weed seed bank because it will allow seed normally below the surface to germinate and be sprayed. This cycle of spraying and cultivation should be carried out until a marked reduction in weed germination is noticed, and should be undertaken in Winter/Spring when weed growth is highest. Weeds that can be spread by soil disturbance like couch and Kikuyu should not be cultivated, but regularly sprayed during their growth season. Weeds should never be allowed to set seed before spraying.

PROS

Spraying or a combination of spraying and cultivation on larger sites is much cheaper than scraping.
No excess topsoil to deal with like scraping.
The period of spraying or spraying/cultivating can be extended at a low cost until weed germination decreases to an acceptable level.

CONS

Spraying or spraying/cultivating can take from about six months to 24 months depending on the weed problem.
Spraying or spraying/cultivating will not reduce soil fertility; important if you want to introduce forbs into an artificially fertilized site.

Scraping

Scraping the top layer of soil from a sowing site is a very effective way of removing the soil weed seed bank. On large sites heavy machinery will need to be used to do this, and the depth of the layer of soil to be removed will be determined by the known history of the site. Where deep cultivation has taken place as in the practices used in a market garden where weeds are regularly rotary hoed back into the soil, a lot of soil will need to be scraped to remove the soil weed seed bank. Sites with a history of no previous cultivation sould only need a shallow scrape.

PROS

Scraping is a very fast way of preparing a site.
If the scrape is deep enough, all the weed seed can be removed.
Extra nutrients caused by stock and supplimental fertilizers will be removed if the scrape is deep enough allowing the addition of forbs to the seed mix.

CONS

Scraping is a very expensive way of preparing a site, especially if machinery has to be used exclusively for site preparation.
If the scrape is not deep enough the site preparation might not be sufficient to give good weed and nutrient control.
Excess topsoil created by the scrape may need to be removed, adding to the cost of site preparation.
Some sites may have a very thin layer of topsoil which when removed reveals a subsoil that is not suitable for growing native grass, or a layer of rock. Test the layer of soil you intend sowing into to make sure it is suitable for your intentions.

Importing soil

This is a method that is usually only practical on construction sites where large quantities of soil are being moved around. Rather than stockpiling the weedy topsoil to be replaced at the end of the project, weed free soil from lower in the soil profile can be used instead. It is important to test this soil to ensure it is fertile enough to grow native grass as some sub soils can be toxic to plants.