Thinking of digging a 500mm trench between garden bed and grass then putting in a type of weed barrier and filling trench with soil. Will this be enough to keep grass out of garden? What type of barrier would be effective?
Kikuyu spreads rapidly through rhizomes and stolons so I'm not sure how effective a trench filled with soil will be. I think you'll find most grasses will spread out and grow over the area where you have dug the trench. The trench system will stop weeds and grass growing up through the weed matting, however, grass not only grows upwards but it spread outwards too. Varieties like Buffalo/Kikuyu are notorious for this and they will throw runners out over your trench very quickly.
Traditionally gardeners would dig in a garden edging to prevent the grass spreading. This allows you to use a line trimmer run hard up against the edging to keep the grass in check. You might like to consider utilising a garden edging in combination with your trench idea.
You should find this step-by-step tutorial helpful: How to install garden edging.
Please let me know if you need further assistant or had any questions.
I used a 380 mm roll of dampcourse. Seems to be working OK so far. Kikuyu roots are insidious, traveling underground down to at least 300 mm. Your 500 mm would be great.
Once you stop the Kikuyu from reinfesting the area, you need to remove any already in the space you are protecting. This is a long and tedious process, as any pieces of root left in the ground will resprout and spread.
Welcome to the Bunnings Workshop community @andring. It's great to have you join us and many thanks for jumping into the conversation.
It sounds like you've had great success in preventing the spread of Kikuyu and I trust @Wy appreciates your suggestion.
We look forward to hearing about all your projects around the house and garden and we'd encourage you to let us know if you ever need assistance or had something to share with the community. I'm sure you'll receive plenty of inspiration for your projects from our creative members as they are contributing their fantastic ideas here every day.
@andring I'm not only wanting to stop the ingress of Kikuyu and Buffalo grass but I also want to protect the timber fence from rotting away. I've been advised not to use plastic sheeting because the plastic will sweat and also allow water to run down behind it. Are you using the dampcourse against a timber or metal fence and if so, have you seen any evidence of insidious moisture damage?
I am using it againt a metal fence sandwiched by a treated pine baseboard screwing it to the bottom rail of the fence. It is working very effectively so far. Perhaps you could have your barrier slightly away from the fence and deal with what comes up between the barrier and the fence. I have not seen any evidence of sweating between the barrier and the treated pine.
The plastic root barriers would be perfect underground to prevent the grass encroaching under the fence. In regards to the raised garden beds, perhaps you would be best off using treated pine sleepers and forming a physical bed to prevent the soil coming in contact with the fence. This would also prevent the weight of the soil from the garden bed being placed against the fence.