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Black Plastic Mulch for Tomatoes

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Black plastic mulch is actually not loose mulch, but rather large rolls or sheets of black plastic film laid over the soil. Holes or slits are cut for tomato plants.

Why use black plastic as mulch for tomatoes? The benefits are many, making black plastic one of the favorite mulch options among tomato gardeners. (Just for starters, check out the wide variety of black plastic options.)

Benefits of using black plastic as tomato mulch

Black plastic heats the soil

Black plastic helps tomatoes even before plants are set out in the garden.

After preparing the soil (tilling it, testing it, amending it, and adding organic material), roll out plastic. Dark colors absorb heat. The soil beneath the plastic will warm much faster than other portions of the garden, making a more welcoming environment for tomatoes. Then when plants are set out in the warmer soil, they acclimate easier, blossom faster, and produce fruit earlier. Warm soil also reduces incidence of damping off. When cool temperatures threaten plants (either midseason or as fall approaches), black plastic keeps heat in and allows warmth to radiate up to tomatoes – so this benefit lasts all season log.

In order for black plastic to have its best effect in early spring, set it out in the garden at least 7 days (preferably more) before you plant tomatoes.

Black plastic keeps weeds down

A layer of black plastic provides an effective barrier to weeds. Light cannot penetrate the dark color (as compared with clear plastic), thereby preventing weeds seeds from germinating and growing.

Black plastic prevents diseases

As water splashes down on the ground, it collects pathogens from the soil (like fungi and bacteria) and splatters onto plants. Black plastic acts as a barrier between the soil and the plant, keeping spores and bacteria from spreading aggressively.

Black plastic prevents evaporation

The soil beneath plastic mulch retains moisture longer and loses water more slowly than exposed soil.  Plant nutrients do not leach out of the soil as easily from the soil, either.

Drawbacks to using black plastic mulch

  • You need to buy it. Other mulches – like oak leaves or grass clippings – are free.
  • You need to spread the plastic each spring and clear it at the end of the season.
  • You will need to replace it nearly every year. Thicker plastic may last more than one season, but don’t count on it.
  • You cannot rely on rain to water your plants. If you decide to use black plastic to mulch your tomatoes, you need to find an alternative way to keep your tomatoes watered. Rain will not penetrate a well-secured black film.

How will you water tomato plants beneath the plastic?

You have several options.  

Tomato Dirt’s top choice: run drip hoses underneath the plastic, positioning on/off switches in uncovered areas.

You can also cut larger-than-normal openings in the plastic for plants, allowing you to insert slow dripping milk jugs at the base or regular watering hoses with holes drilled where tomatoes are placed.

Another option is to run two parallel sheets of the plastic along either side of the row of tomatoes, leaving a strip of soil exposed lengthwise between plants. Place a soaker hose along the exposed soil.

What black plastic mulch should you use?

Plastic mulch film is available in rolls which are 3-feet, 4-feet, and 5-feet-wide. Thicknesses can vary and range from 0.5 to 1.25 mm. Thicker plastic withstands wear and tear and may even last more than one season. Check out all kinds of options and choose the black plastic mulch that works best for your situation.

Two special types of black plastic you might want to consider when mulching tomatoes:

How to lay and secure black plastic mulch

To get best results, apply black plastic before planting tomatoes. You can also set it out well into the season, especially to keep weeds down, but be prepared for a bit of awkward maneuvering between plants.

  • Prepare soil beds for tomatoes.
  • Arrange drip hoses along the row, allowing the hose ends to protrude on the ends.
  • Unroll black film. When applying mulch on a windy day, walk with the wind rather than against the wind while unrolling the film, keeping the roll as low as possible to prevent sheets from becoming airborne. 
  • After arranging the plastic over the area, secure the edges with ground staples, bricks, or rocks. If you have raised beds with wood borders, you can staple edges to the sides of the beds.
  • Measure and mark the planting holes for tomatoes.
  • Allow black plastic to warm soil for at least 7 days before cutting slits or holes in the plastic to plant tomatoes.

An extra tip for securing black plastic mulch

Prepare a raised row for your tomatoes, leaving a trench on either side.  Spread the black film over the lengthwise mound and bury its edges in the trench bordering both sides of the row. Burying the edges keeps the film secure. Keep in mind that this method allows for very little air circulation.

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