Interesting problem. My gut instinct on this one is that it's nutritional, and I would suspect first and foremost maybe a calcium deficiency, or more accurately, a calcium availability problem -- it may be there in your soil, but chemically "locked up" due to the soil pH. Of course, it could be related to some other nutrient as well, perhaps boron or magnesium as well?

Here's a link I found that might help:

http://www.ces.ncsu

That is an interesting dilemma, as broccoli is normally easier to grow than cabbage or brussel sprouts. Soil diseases are a possibility but they would normally get cabbage or Brussels Sprouts quicker than broccoli. Both broccoli and cauliflower will button ( form coin sized seeds when the plant is quite small) if allowed to get rootbound before transplanting. But it seems you are planting 6 -8 weeks old plants. Are the plants getting appropiate size? have you pulled one up and looked at the roots of mature plant? Do they form decent side shoots after you cut the main head? http://www.organicgardening.com/feature/0,7518,s1-2-9-1463,00.html http://www.professorshouse.com/your-home/gardening-plants/planting-broccoli.aspx
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How much wood ash did you incorporate when you tried? What is the pH of the soil? I assume the N is OK, else you would not be able to produce cabbage. I also concur that you should pull up a plant or two to look at the roots.

Definitely check your soil pH in the top 6-8".

Also, Broccoli are notoriously sensitive to abrupt temperature changes after planting out. If you have an unexpected warm spell, or mild weather is followed by a near freeze in the first weeks, the broccoli will tend to make those stiff button heads.

When I have tried to grow broccoli and cauliflower, I get wonderful looking initial plants, but when they start to set the flower heads, they spread out in an open looking flower as opposed to neat closed tight heads that should grow in circumfrence. Could this be a water problem or am I doing something else wrong?
This year if I try to grow them I will do so in a raised bed to better control water, etc.

Thanks for all the suggestions! My plants grow to a decent size and the leaves are a healthy dark green color; it's just the heads that seem to have problems.

I'm guessing that my problem is the abrupt temperature changes--spring tends to come in here with sharp temperature swings, and there are sometimes freezes (and sometimes fairly summery days) in the second half of May when the broccoli are newly out. Would I do better if I covered them with ag fleece to moderate temp swings? or if I started them later and put them out in early June when frost danger is past?

If you think that is the problem, you are better off making them a fall vegetable. If they are more mature they will withstand the swings better. Plant them in June for consumption in October.

I also can't grow broccoli to save my life in the spring in SE Wisconsin. I can grow brussel sprouts, cabbage, etc. Springs are just too inconsistent around here. 80F one day and snowing the next.

I almost always get better crops when planted for a fall harvest than planted for a summer harvest (except the warm season crops, of course).

Besides weather temperature problems soon after transplanting, I suspect that the biggest problem is not setting them out while the plants are young enough....I like 23 days from seed sowing. Try it...you'll like it.

If you are getting normal healthy plants buttoning is not the problem. I suspect you are getting insect damage, when the head is forming. Check for aphids, harlequin bugs, even cabbage worms can do a number just as the head starts.

How far do you space the plants ? spacing is critical for broccoli, for large heads space 10 inches, closer spacing will result in higher yields but smaller heads. hop this helps. bill