Thanks for the feedback, sounds like time to go buy a pressure tank.
What is the size tank I would need for the following:
pump - 1.5 hp
lift from lake - 8 ft.
elevation push from pump to sprinklers - 35 ft.
6 sprinklers with output I estimate @ 2 gpm each
distance from pump to sprinklers - 150 ft. max
JHV
The correct answer to your question depends on WHICH 1.5 HP pump you have.
If you have a 1.5 HP centrifugal that is often sold as an "Irrigator" or irrigation pump, and you are using 6 sprinklers that use 2 GPM each, then you would need a big tank to be effective as a storage unit.
If you are using a 1.5 HP shallow well jet pump it might be ok if you set the pressure switch high enough.
You should be looking at the pump curve, which is a graph of pressure on the vertical scale and GPM on the horizontal scale. Go up from 12 GPM and see what pressure that gives you.
What you should really be doing is looking at that curve to determine what the flow.
The Goulds GT15
http://www.goulds.com/pdf/7305.pdf is a 1.5 HP "Irrigator" pump and will deliver 25 gallons per minute at about 30 psi for your suction lift and sprinkler elevation condition. It is really about a 40 GPM pump, and while it will operate at 12 GPM the pressure for your system may not be satisfactory. Most other 1.5 HP irrigation pumps have similar characteristics.
If you have a jet pump it will be better match to your 12 GPM and pressure requirements.
If you are operating the pump solely for irrigation (watering the flowers with a hose and a nozzle that is being turned on and off is not irrigation) you probably don't need a pressure tank or switch. Just make sure that there are no shutoff valves in the line and turn on the pump when you need water.