Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu and Emmalea Ernest, Scientist – Vegetable & Fruit Crops; emmalea@udel.edu
High temperatures (90°F or higher) coupled with clear skies can lead to heat buildup on the surface of black plastic mulched soils. We have found temperatures of over 140°F at the surface of black plastic mulch. This can cause losses with transplants because stems near the mulch are damaged by the high heat. In crops seeded through the black mulch, germination is often reduced, and if plants do emerge, they can be killed by the excess heat. Another problem is high soil temperatures under black mulch which can lead to fruit quality issues in tomatoes and peppers. In onions, black mulch can cause damage to bulbs due to excess heat.
Late spring planted peppers are very susceptible to stem heat necrosis on black plastic mulch (Fig. 1). This is where the high temperatures at the mulch surface causes damage to the stem, often causing plants to collapse. When daytime temperatures are in the high 90s, the surface of black plastic mulch can be as high as 140°F, which will kill plant cells.
Figure 1. Pepper transplant with stem girdling from heat necrosis
There are several strategies that can be used to reduce stem heat necrosis. Larger transplants with thicker stem diameters are less susceptible to damage. Make a larger hole when transplanting and make sure the plastic mulch does not touch the stem of the transplant. White particle films (clay or lime based) sprayed at the base of plants over the mulch can also reduce plant losses to heat necrosis. Putting a small mound of clean sand around the plant stem will also eliminate this problem.
Shade cloth is another potential strategy for reducing stem necrosis. In a 2022 trial conducted by the UD Extension Vegetable Program, 30% black shade cloth was very effective in preventing pepper transplant loss. The shade cloth was applied on June 1, the same day as transplanting. In the shaded plots there was 97% stand 49 days after transplanting; however, in unshaded plots only 64% of the transplants survived. The shaded treatment also had larger plants (Fig. 2) which eventually produced significantly higher marketable yields.
Figure 2. Pepper plants that were shaded immediately after transplanting (top) had higher stand establishment and greater plant vigor than unshaded plants (bottom).
Switching to white plastic mulch for later spring plantings can reduce losses significantly (white plastic will be 10-20 °F cooler than black plastic mulch). White mulches can lower bed temperature by up to 20°F. Use of white mulch increases transplant survival and increases germination and survival of seeded crops. The cooler soil can also increase root function and reduce fruit disorders such as white tissue, blotchy ripening and yellow shoulders in tomatoes and blossom end rot in tomatoes and peppers.
In onions, cutting the black mulch in mid-June as bulbs are increasing size has been shown reduce to reduce bulb damage.
In the past, a rule of thumb has been to switch to white mulch in the middle of June when days are longer and air temperatures are higher for longer periods of time. White mulch should also be used for crops planted in July and the first half of August.
The most common mulch used is white on black. The black side reduces weed germination, and the white top reflects solar radiation thus cooling the surface and the soil beneath.
Is there an advantage to switching earlier? Up to the middle of May, black plastic (or other soil heating colors) should be the preferred mulch to get warm season vegetable plants off to a good start when soil temperatures can be variable and bed heating improves crop performance. The second half of May can see some very hot weather as can the beginning of June, but this varies from season to season. Past research has shown no benefits to using white mulch in this period and often reduced crop performance in warm season crops such as watermelons. If long range forecasts are for warmer than normal temperatures, laying white or reflective plastic earlier in June may be advised for sensitive crops.
White mulches have also shown benefits in spring and summer planted cool season crops such as broccoli, lettuce, onions, and day neutral strawberries planted in April.