Express your concerns to your landscaper and ask them what they do if plants die. If they offer a guarantee, get it in written form.

and with irrigation, it's not likely that plants will die.

You are right to be concerned, and you should review your written contract to see what replacement guarantees are stated, and for what period. Installation during summer, even with irrigation, can be stressful on plants, especially if they aren't well watered before installation, or you get an extreme heat wave during the first couple of months, or the rains aren't regular. You will definitely want to keep an eye on the weather, and also closely keep tabs on the plants. If you have no guarantees, or for too short a period,(6 months for shrubs and 1 year for trees is customary here in California), then it might be better to delay installation until fall if you can.

Thank you for your comments.

They guarantee their work for two years. But you are right, I will get them to put it in paper.

Having the guarantee in writing is necessary as an oral guarantee might possibly be equal to zero years of protection. (Actually, two years would be unheard of in any place I've ever worked.)

What's the likelihood that landscape installation companies--who must guarantee the work-- close their doors and go out of business during the summer months? Zero. Would they continue to work if it meant that all their efforts must be done over again in the fall? Not hardly. It is irrigation systems that make planting all summer possible. The idea that landscapes with irrigation systems ALSO require regular rains in order for plants to live is nonsense.

Irrigation does allow planting in summer, but things can malfunction. It happens. if a certain sprinkler is misdirected or otherwise not functioning properly during a heat wave, some plants could be lost. Best to have "promises" in writing.