When you irrigate the drippers will wet the soil in a expanding "pie" shape. At 12" spacing the wet spaces will meet at 6" radius from the drippers and then the wetted width will be 6" on either side of the driptape. So it would be a good idea to plant about 6" to the side of that driptape. However, much more is influenced by the type of soil you have. If you have less sandy soil then the wetted "pie" will continue to spread more as you water longer instead of the water draining downward. You could then plant further away from the tape with no problems.

You should have no problem in pressure loss with only 40' long rows with that dripper spacing either. Your plan seems OK except for a few suggestions in your management of the layout: Do you usually plant corn rows that close? Regardless I find that cultivating around driptape can be a problem unless you either move it frequently or put mulch down to prevent most weeds. You will then benefit from less evaporation of the water since the tape will be under the mulch.

I should point out that I never drip irrigated the crops you mentioned except for tomatoes, squash and cabbage. When I do drip irrigate I almost always use driptape under black plastic mulch for transplants of any kind. If you sow seeds for the other crops you mentioned then plastic mulch is not often used. What may work best for you is to either remove the driptape to allow cultivation of those 2-row/tape crops and then relay it or to position a single driptape tight against each individual row and avoid cultivating close to it.

The relative point I wanted to make was in planting multible rows of bulbing onions and leeks on each 3' wide black plastic row. I'll usually prelay two driptapes (16" apart) and lay the plastic over them. Then I transplant the onions and leeks, 4 rows/plastic row , planting each row 4" on either side of the driptape. The plants perform very well in that spacing.

We drip irrigate everything, radishes, carrots, tomatoes, broccoli, etc. I do not know what kind of main line you are going to run (poly tube or pvc) but I will give you the upsides and downsides to both. The poly tube main line is cheaper, that is the only upside to poly tube that I can think of. The down side is it is more work to install. You have to punch holes in it, which means if you want to rotate your plants next year with different spacing between your rows you need to use goof plugs to plug the old holes. Next thing is, if you consider your garden an extension of your backyard as I do, it doesnt look as nice.

PVC is more expensive but well worth it in the end. It looks nice and professional. My husband spaced the tees coming off the main line every 6". That way when we decide to rotate our plants we have pvc plugs in the rows that are not used. Looks a more professional, which is what he was looking for.

As far as planting 2 rows down the side of the drip tape, it works fine, but in my opinion you are not utitilizing the drip tape at its full potential, meaning you are using more water than what needs to be used. If you are on a water meter that is money out of your pocket. If you are on a well and water can get scarce during some parts of the summer then we like this way the best. Example of water use, lets say you plant radishes, they only need to be a few inches apart. When you use 12" on these rows the water has to be on for quite awhile to get all of your radishes in between, but if you used 4" drip tape on these rows as I do, I turn the water on for about 10 minutes because all my outlets are set near my plants. When you use 12" for your tomatoes, you are probably going to skip some outlets, make sure you plant something in the outlets you skipped, like lettuce or peppers, that way you do not waste the water and get more out of your garden. If you would like anymore information on this please feel free to email me at charleene@gisco.net.

Getting ready to lay drip tape on our veggie plots as well. Where did you get 4" spacing tape?

How's the PVC hold up over the years being exposed to the sun?

I can see cultivating around smaller plants being a pain in a multi plant row/bed situation without mulch. Not sure how to deal with that. I'm not ready to invest in a plastic layer.

bmoser, how do you irrigate your other crops that you don't use drip tape for? (Just checked out your website...great work!)

fventura,
I got the 4" at tcdrip.com. It comes in different lengths. I have pvc in my garden but I have only had it a couple of years so I couldnt say how it holds up, but I did read in one of the forums that said if it is above ground then pvc should be used, it will not get crushed as easy as poly pipe. I would figure, that being a harder plastic it would hold up for many years. Using poly pipe or pvc is up to the gardener, if you want irrigation that just does its job and dont care about what it looks like then polytube is the way to go. I have raised beds and want them to look nice and professional so my husband put in pvc.