Warming soil in your garden using plastic mulch

What’s so good about plastic mulch?

Plastic mulch is a good way to get a head start on spring because it warms the soil. Unlike organic mulches, plastic does nothing to enrich your soil. But, plastic mulch prevents soil from drying out and controls some weeds. Plastic mulch is a barrier between the soil and the plants themselves, which helps to prevent some soil-bourne diseases.

Many farmers use plastic mulch. The University of Alaska has shown that:

“…cabbage and broccoli grown with plastic could be harvested 7 to 10 days earlier than plantings with no plastic. For crops such as zucchini squash, harvest may start 10 to 14 days earlier with overall yields 2 to 5 times greater when grown with plastic mulch and row covers. In trials conducted in Palmer, Alaska, the yield increased 8 times for cucumbers in a mulch and row cover system compared to the adjacent field.”

These are impressive results. Using plastic mulch improves yields (the number of vegetables per plant), with earlier harvests and better-quality vegetables.

Plastic mulch is good to use in the home garden. You can pair using plastic mulch with raised beds, crop covers and/or tunnels for even better results.

What kind of plastic?

You can buy special plastic garden mulch at garden centers or on-line. Rolls of plastic such as vapour barrier used for building, or plastic used for wrapping available at hardware stores can also be used.

Home gardeners can recycle sheets of plastic to use in small garden plots. Plastic wrap used in packaging or shipping, or even heavy-duty clear garbage bags can be used. If the sheets are too small to cover the garden, they can be pieced together using clear packing tape. The thicker the plastic, the better. Thin plastic tends to tear easily.

You can use any kind of plastic you have on hand but some colours of plastics are better for different things. Clear or black plastics are the best choices.

Clear plastic

• Warms the soil better than any other colour of plastic: a great choice for the far North
• Poor at suppressing weeds

Black plastic

• Warms soil but not as well as clear plastic
• Very good at suppressing weeds

White plastic

• Keeps soil cool because it reflects light - not recommended
• Very good at suppressing weeds

When should I lay down the plastic mulch?

Install the plastic mulch as soon as the snow has melted. Allow the soil to dry slightly. Wet soil tends to compact easily. Compacted soil makes it harder for plant roots to grow and take up water and nutrients.

How to tell if your soil is too wet: Take a handful of soil and squeeze it in your hand. Open your hand and press your finger against the ball. If the ball feels wet and holds its shape when you press it with your finger, then it is too wet. Let the soil dry out before you prepare the soil and install the plastic mulch.

If the ball falls apart when pressed, it is dry enough to prepare the soil and install mulch. Apply water afterward.

How to install plastic mulch

1. Prepare the garden bed: remove grass, weeds or other plants. Lightly fork in compost or other organic matter. Rake for a smooth flat surface. This can be done in the fall.
2. Apply fertilizer if using. Spread granular fertilizer over the surface of the soil and lightly scratch in.
3. If you are using drip irrigation, install it on the surface of the soil near where your transplants will be planted.
4. Tightly stretch the plastic over the surface of the bed. Secure the edges of the plastic by burying with soil or weighing down with rocks, planks of wood or anything you have on hand to prevent it from blowing away. If not secured, winds can tear away the plastic and litter the environment. Make sure that the plastic has good contact with the soil and is not loose.

How do I plant my transplants?

Make an ‘X’ shaped cut in the plastic so that there is enough space to dig a small hole big enough for your transplant.

Another way is to make a round hole in the plastic using the sharp end of an empty tin can.

Plant the transplant in the soil under the hole in the plastic. Water well under the plastic.

What if I am direct sowing seeds?

In this case, use clear plastic temporarily. Remove the plastic when it has done its job warming the soil. Then go ahead and sow your seeds.

How do I water my plants?

Drip irrigation is a good way to water plants when the soil is covered with plastic mulch. Drip irrigation sends water to plants through a series of plastic tubes and emitters. The system is fed by water from a hose which is controlled by valves. Drip irrigation systems can be purchased at hardware stores, garden centers or on line.

If you don’t have drip irrigation watering is a bit trickier. You will have to pour water into the holes you made for your transplants. Try not to let the water flow on top of the plastic. On the plus side, you may not have to water as often because plastic mulch prevents the soil from drying out. As transplants grow bigger, you can make the holes a bit larger to make watering easier.

When should I remove the plastic mulch?

You can either use plastic mulch temporarily in spring to warm soil, or keep the plastic mulch on the soil until the end of the growing season. When harvest is finished, remove the plastic mulch in the fall. Take care not to work pieces of plastic into the soil. Plastic tears easily and will blow off the garden and into the environment. After removing the plastic mulch, this is a good time to put a thin layer of compost on top of the soil. The compost will protect and nourish your soil.

Sources

Karlsson, Meriam. Plastic Mulch, Row Covers and Low Tunnels for Vegetable Production in Alaska (2017). Publication No: FGV-00647 Cooperative Extension Service, University of Alaska, Fairbanks.

https://extension.usu.edu/productionhort/files-ou/Use-of-Plastic-Mulch-for-Vegetable-Production.pdf