During summer water the plants in the morning or late in the afternoon. This will avoid water to get evaporated. Since the potted plants cannot go deep into the soil in search of water, water your potted plants more than usual to make up for the absence of humidity. See to it that the soil is moist most of the time. This will keep them healthy throughout summer.
To retain the moisture of the soil mulch it with wooden pieces, twigs, leaves, etc. This will help create natural fertilizer for the plants. If the soil in the pot is sandy, mix it with the right amount of clay. Clay will also help retain the moisture.
Keep the potted plants in a cluster. The plants will lose water from external sides and will face the sun only from one side.
Prune the plants and get rid of unnecessary foliage. This will avoid plants to lose water through transpiration.
Provide the plant soil with more fertilizer in summer to sustain their growth and extra energy requirement after the winter season. Uproot all the plants from the plant soil, for the nutrients and water to be utilized only by plants and not weeds.
With these measures, your plants will remain hydrated throughout the summer season.