Dear Judy,

Thank you for contacting Ask Extension about protecting your vegetable garden during this extreme heat event. Here are steps you can take to help your plants:

Mulch - Almost any organic mulch is better than none, but 5-inches of straw mulch is probably the best choice. You can also use wood chips, dried leaves, grass clippings that have had no chemicals applied, or even shredded cardboard.

Shade - Potted plants can be moved to a shadier area. Fixed beds will benefit from a protective cover. Use 30% shade cloth, which will create 30% shade and allow 70% of the sunlight to pass through. (Heavier shade cloth will deprive plants of light and may result in sunburn when the cloth is removed later in the season.) Alternatively, a floating row cover (sometimes referred to as Remay fabric) will provide some protection also. If nothing else is available, you can "tent" cardboard over the plants during the hottest part of the day. Do not use plastic or a tarp. These can harm plants by intensifying the heat regardless of the color.

Water - A drip hose or hand-watering will save water by reducing exposure to wind and evaporation. Apply water early in the morning if possible. You can also water in the evening but try not to get the foliage wet since this can lead to fungal diseases, especially on tomatoes and peppers. The amount of time needed to wet your soil to the appropriate depth will vary depending on your soil type and the way you deliver water (by hand or by irrigation system). The amount of time stays the same in all seasons. However, the frequency you must water will change by season. Water is needed more frequently (but not for a longer time) during the summer, and less in the spring, fall, and winter.

Cultivation - Do not try to plant or transplant outdoors during extreme heat. Harvest during the cool hours of the day when the plants hold the most moisture. 

This article may provide some additional information:  
https://extension.psu.edu/heat-proofing-your-vegetable-garden

I hope this information is helpful. Please write again if you have other questions.