Smother weeds with cardboard is a great way to start a new garden patch with minimal work. Cardboard that is plain and thick will smother weeds as quickly as 3 weeks but should be left on the soil to break down for at least 3 months to make sure the weeds have been killed off.
Soft winter weeds will die off quickly but stronger weeds like thistles with deeper tap roots or onion weed with bulbs will take longer to kill off. Kikuyu or spreading grasses like buffalo will also take longer to kill off. It is important to cover them with thick cardboard, mow them down low and cover them with a thick layer of compost or mulch.
This article will explore how long it takes to smother weeds with cardboard and my easy tips to make sure that the weeds are killed off as quick as possible.
How long it takes to smother winter weeds with cardboard
Winter weeds will be smothered under cardboard in as little as 2 weeks. The cardboard will block the sun preventing photosynthesis and these soft weeds die off quickly. I would leave cardboard over the top for at least 3 weeks and check to see if the green color is gone.
Even better, layer compost and aged manure over the top to create a new garden bed. The weeds will die underneath the cardboard and worms and soil bacteria will break them down.
Winter weeds are soft, easy to pull out and usually bright green and full of water. These include chickweed or soursobs. They are easy to pull out by hand but are even easier to cover with cardboard and smother for a new garden bed.
These weeds will die off quickly if the sunlight is blocked by the cardboard. Make sure that the cardboard is laid over the weeds and there are no gaps for weeds to grow through.
It is important to cover the weeds before they set seed to reduce the likelihood of them growing back when the cardboard eventually breaks down.
How long it takes to smother grass with cardboard
Lawn or grass can take 4 weeks to 3 months to be fully smothered by cardboard. Trailing grasses that grow from nodes like kikuyu or buffalo are harder to kill off with cardboard. They can lay dormant for months under the soil and are ready to grow back as soon as they get light and water.
Before covering grass with cardboard for a new garden bed it is best if you can mow it down as close to the ground as possible with your lawn mower. Then you can layer the cardboard over the top of the soil and grass remnants.
If you are planning on covering grass or lawn with cardboard for a new garden bed it is essential to cover the area thoroughly with the cardboard. I would add 2 layers of thick cardboard to make sure that the grass has been completely smothered.
How long it takes to smother weeds with tap roots with cardboard
Larger weeds that have deep tap roots take more effort to kill off with cardboard. Deep weed like dandelions or thistles have a long root that reaches deep for water and nutrients. These weeds can be killed off under cardboard in 4 weeks if they are small.
If the weeds are larger and the cardboard is not sitting flat on the soil then it is better to pull them out first. Make sure that they are lifted out with a garden fork or grab them with gloved hands close to the soil and pull gently. This will help the whole root come out from the soil so they can’t grow back.
Tips for covering weeds with cardboard successfully
Here are my top tips for covering weeds with cardboard to kill them off fully so they won’t grow back. This will give you the space for a new garden bed or stop weeds coming up in your flower beds or vegetable patch.
1. Use thick cardboard
To make sure the weeds are fully covered and all of the light is blocked I like to use good quality, thick cardboard. This means packing boxes or heavy duty freight boxes that are almost twice as thick as regular shipping boxes.
Thin cardboard like cereal boxes breaks down fast and can allow weeds to grow through. Choose good quality, plain cardboard boxes and remove the tape. This type of cardboard will be slowly broken down by soil bacteria and worms and will help to improve the soil underneath.
By the time that the cardboard has been broken down the weeds underneath will have died off. If you are using the lasagna method or making a no-dig garden then you should have a weed free space for your new plants to grow.
2. Cover all the gaps
When layering your cardboard on the soil, make sure that you move the pieces close together and don’t leave any gaps. Any small gaps where light can get through can allow weeds to continue to grow. Large boxes work well because they will cover the area thoroughly.
Overlap the edges so that each box traps the other in and covers the soil entirely.
3. Double layer the cardboard for persistent weeds
If you are layering cardboard on the ground to cover tough weeds or lawn then I would always go for a double layer of cardboard. Lawns are hard to kill off and can find even the smallest amount of light to grow back.
A double layer of carboard is usually needed for grasses that grow with runners or have lots of seeds. Double layer the soil with good quality, thick cardboard. You can then layer compost and aged manures over the top for an instant garden bed.
I have used this method to create my new fall flower bed. This is full of violas which are ready to grow over the cooler months and fill my winter garden with color.
How Long it Takes to Smother Weeds with Cardboard | Summary
Cardboard is a great product that will break down naturally into your soil, smothers weeds and feeds worms. Recycle old packing boxes and create a new garden bed in a day. The weeds underneath the cardboard will be smothered in 3-4 weeks but the carboard will help to protect your new soil for a month or two after this. Choose good quality, thick cardboard and prevent the weeds from coming back.
I am an accredited practicing dietitian, experienced gardener and a dedicated cook. I love writing and sharing my experience so you can learn from my successes and mistakes.