Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@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.
One solution is to reduce bed temperatures by using white mulches. 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 also can 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 this 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 planted cool season crops such as broccoli, lettuce, onions, and day neutral strawberries planted in April.