I have never done anything in a lawn. I suspect that the competition with the grass roots may well be a problem. I think the specific issue with the hole may also be a smaller problem. I think it would be wise to remove a little more lawn around the hole. The squash will shade it out anyway if they grow.

Just wondering why you chose to plant them in the lawn?

I ran out of space and had success with accorns (got 4 after planting in July) the year prior.

Hey they only need soil, so it didn't seem to matter what was on top of the soil. Punch a hole through the sod, widen and deepen the hole below the ground line and fill with some nice compost.

This seems like an interesting way to steal space, why let them negate that much ground of your garden? Along with growing cukes and beans vertical this seems like a good way to stretch your yield. I have no experience with pest growing on lawn vs soil yet, we'll see. Last year it didn't seem to bother them to rest on the grass.

I guess alot depends on what your soil type is. If your digging a hole into primarily clay soil, you risk creating a "bathtub" effect. What happens here is that water runs into the path of least resistance (the nice compost hole) and then fills up that hole because it cannot drain properly through the clay surrounding it. The roots will be suffering if you get too much water. If it is dry, this won't be as much of a problem.

You will need to ammend that compost to provide the necessary nutrients. Compost by itself will not have sufficient nutrient value for the large vines and you cannot count on much nutrient value in the ground surrounding it if it has been grass for more than a few years.

Another challenge will be cutting the grass under the vines. Unless you have grown pumpkins before, you might be in for a bit of a shock as to how big they get. You can easily expect a well grown pumpkin vine to take up a circle of 15 feet in diameter quite easily. Unless you are cutting the vines with the lawnmower (then what is the point of growing them in the first place?) you are going to have a wild patch of prickly vines, leaves, flowers, fruit, weeds and grass. Pumpkin patches are not "tidy and attractive" unless you heavily, heavily mulch the area. It doesn't bother me, but my pumpkins grow in the garden out back. You may not really like how it looks on the lawn.

If you still decide you want to do this, then I would think 1 plant per hole will be the best. There will be intense competition for water with the thick layer of turf, so spacing in a dry garden environment for pumpkins is usually 6 feet between hills. "But I will water the hole" you say "so I won't matter". This will provide the base with moisture yes, but the root system of the pumpkin is absolutely enormous. If it is extending into the surrounding ground, and the surrounding ground is topped with an established, matted turf, that ground under the turf will be quite dry. The roots will need to wander long and far looking for sufficient water and nutrients.

When deciding where to plant pumpkings, just remember the old saying: where do you let a 500lb gorilla sit? Anywhere he wants to. The same applies for pumpkins: you can't just plop them anywhere and expect them to cooperate.

Hope this helps,
Michael

I generally put in about 5 seeds per mound. I build HUGE 2 foot mounds, very loose, rich soil. Then I water and feed the heck out of them all during the growing season. This has netted me more pumpkins and squash on less space. I think the main reason for this is that some vines are stubborn about putting out female flowers and if you put in only four plants and ALL are stubborn, you will have less product. I over-plant and that way I get at least half of the vines that are good female flower-producers and guarantee I get more pumpkins this way. Yes, it takes more water and more food.

(BTW, easy to tell which are male and female flowers, probably find this in another thread, but I'll blather on anyway! The males have very thin, spindly, long stems. The females are squat, short stems and usually have a fat little place right below the flower, even before they open. This will grow into the pumpkin. I'm not sure, and I'll bet my crop that I will be corrected here, but I think this will grow into a smaller pumpkin even if it's not pollinated, but will not have any seeds or not have viable seeds.)

I often trellis the squash (and even pumpkins on occasion) and you get much nicer, cleaner product with no flat places or that unripe spot that always seems to happen otherwise. I've made little hammocks for them in some cases. Last year, everyone was very amazed at my pumpkins that I set up on growing trellises with supports as they were nearly perfect in every case, not a spot on them. The vines are very strong and can easily support the weight as long as the connection to the vine is strong. This makes it easy to grow a lot of pumpkins in a very small space, I just wind the vines around and around the trellis.

I've also had better luck with pumpkins in full, all-day sun. If there's slight shade or part-time full shade, they simply do not do as well and you don't get as many.