It's probably best to check the instructions on the packet of each. If there aren't any, 16-18 inches for your biggest ones should be sufficient. Planting them one seed at a time will probably be best unless you want to be sure it sprouts. In that case, plant 2 or 3 seeds but remove all but one seedling. You'll get better plants when they aren't competing for soil and water. Happy Growing!

Best of luck with this, I am growing sunflowers for my daughter for the first time also. Here are my suggestions.

Get 1-2 bags of compost / soil conditioner and spread it over the area where you plan on growing. If you have not amended your soil in the past this will give it a good head start. Add another bag or two in the fall to prep the area for next year.

Since it is late in the season you should not have to worry about them germinating but next year, I would start direct sow in early April do get a head start. My sunflowers have a maturity time of 90 days.

I had one grow about two to two and a half feet wide and as tall as my house but it was a single planting with no other plants around it. I can't remember what the variety was though. It was crazy huge. We started seed in pots, dug a hole in the ground twice as wide as the plant, plopped the plant in the hole, watered around it before placing the dirt back in the hole and top dressed it with that cheap bagged cow manure from Lowe's then watered again.

We planted ours in mid to late april. The largest are about 6-7' right now. Just amazing! Earlier this week they were about 5.5'. They sprung into action after some rains we had Monday night!!

Thanks everyone for your responses. I'm kind of excited about this. Like I mentioned we do have a very large area for planting and with a large assortment of seeds I hope they do well. If I have any luck this year I will definitely plant sooner next year. Thanks again.

I got one sunflower now that is about 6 inches tall.
Hope it keep going.
I had about 15 to 20 plants but the deers ate them and part of this one too (that the reason it is so small).
I spray it with deer off now.

I've already been able to cut off one sunflower head full of seeds for the birds. I decided to do now it mainly because it was in with a bunch of "wildflower" seeds and was hogging a lot of the water, plus it was out of scale with the other plants. Mine were seeded in April, too.

Sunflowers are in my top five of flower favorites. Summer is not summer without them. I actually plant my seeds in a box so I can control them. Then I intersperse them in the perennial garden. Autumn Sunset, the branching type, is wonderful for the goldfinches to dance around on. They are just on the verge of blooming. I think I planted the seeds in April.

Sunflowers make a dense matt of roots and therefore they don't like having other plants too close - they want all the water and nutrition availble. The big mammoth ones can grow really fast and get really tall. People that grow them for competitions build scaffolds to support them - often over 15 feet tall!!! keep that in mind -they tend to fall over if not supported.

I would only plant a few seeds of each kind otherwise you will run out of room.

I would plant them in the richest soil you can find or buy and be prepared to feed the big ones (I feed a half strength of soluable plant food like MiracleGrow each week for my heavy feeders). Not feeding them so much will keep them shorter. It won't affect the flower performance but the plant will have more yellowing or deteriorating leaves by the time it gets to blooming. For perfect looking plants and flowers you kinda have to feed these guys (of course in the wild they do just fine without any help!!!)

You can help keep down the weeds by mulching heavily around them but don't let the mulch go right up to the stalks.

Planting them now will give you flowers at least by Sept. If you plant some again in mid July you'll have flowers around mid October. So you might want to sow a few seeds of each type every two weeks so that you have flowers from now til frost.

I like sunflowers and have grow some but Don't like the problem with them breaking down. I see pictures from time to time of big fields of sunflowers and they not falling down.
Why they stand so good? Here is a good pictures of some in Denver.

Here is a link that might be useful: Sunflowers in Denver, Colorado

Basil, I think it might be because of rainfall when the young plants are developing their root systems. Farmers will know when best to plant while the home gardener sometimes waters too much and the resulting root system of this tall plant isn't as deep or wide spread as it would be if it had to search for available moisture.

and, Basil, those large stands of one crop help keep the wind up and over it. You'll probably see lots of fallen down plants around the edges of the field.

The sunflower garden is coming along nicely. Still in the very early stages but it looks like all the seeds I have planted they are coming up. I've got so many different types of flower seeds planted. I'm anxious to see how this garden will come out. Actually when my mother in law passed away a few months ago the funeral home had these packets of seeds with her name and date of death on them. They were suppose to be for all the people who came to the funeral home. There were very few people that actually came so we had lots of seeds. I also planted all I had in the garden and it looks like they might be a wildflower mixture. Is this something that funeral homes do? I was surprised at this.

Any comments on the funeral home?

I think the seeds were a good ideal.
Should be done more often.

I keep having problem with something eating my sunflowers all the way to ground. Don't think it is a deer. I may put a fence around the area (not very big group). Do groundhogs eat garden plants? I have one sunflower that about 2 feet tall that may have a flower before long (in another part of my park).

Yes, groundhogs/woodchucks eat plants - at least certain ones. Our resident woodchuck, APRIL, eats my tomato plants, but she doesn't eat the sweet pepper plants. Nancy

I think I have created a nightmare here. The garden is coming along nicely. Can't wait for flowers but before we planted we rented a tiller. Now, not only do we have our flowers but I have never seen so much crabgrass in my life. It's taking over. Is there anyway of killing the crabgrass without harming my flowers? What can I do?

When you till the soil it uncovers the years of grass seeds due to neglect. For example each grass plants makes lots of seeds, and over the years that the grass is allowed to seed it all builds up in the soil. When the soil is turned new seeds are uncovered.

The only way to to keep turning and killing the new sprouts and do this for a long long time. So normally you can not go ahead and plant your flowers seeds in an area with weed neglect.

You could try to start with big flowers in pots you buy at the nursery, then remove all the weeds when they are small.

You can remove the soil and replace with sterile soil mix or you can build a wooden planter box and top sterile soil mix on top.

I just tuned in the see if a sunflower would like rich soil and I guess it will.

I use cardboard to keep weeds from popping up in my beds. I put it down on a windless day, soak it with a garden hose and spread straw or mulch on top of it.