My Ambrosia usually gets to around 5 1/2 to 6 1/2 feet tall. I can usually see tassels on shorter corn too, but by the time the whole plant tops out it ends up about right. I space mine differently than Jal, but the result is about 2 square feet allowed per plant. Yeah, 68 days seems awfully short. My Ambrosia package says at least 75 days.
Getting back to weird weather, forecast for Mon, Tues, Wed is for 100+ degrees. Yet, June 13 we had a low of 27 degrees. Lost 2 tomato plants, some freshly sprouted squash, a few pepper plants; potatoes got hit again, some bean sprouts lost, and even a couple of corn plants are crunchy now from being frozen. I'm dangerously close now to not having enough time for winter squash to mature before fall frosts.
All this talk of planting after the last frost is well and good, but what if there is no last frost?! So after the coming heat wave, is the jet stream gonna take another dive and freeze more stuff? Sigh...
Luckily, at my lower elevation garden it didn't freeze.