Buy the 2015 Old Farmer's Almanac!
Hi Don, Hard cores inHi Don, Hard cores in tomatoes are usually caused by weather related stress like low overnight temperatures early and late in the growing season. Provide consistent warm temperatures during cool nights by using row covers or other protection. Too much fertilizer early in the growing season can also cause hard centers. Meatier tomato varieties have less core and most modern hybrid tomato varieties are made to be as coreless as possible.

2015 Special Edition Garden GuideCooking Fresh with The Old Farmer's AlmanacThe Almanac Monthly Digital MagazineWhat the heck is a Garden Hod?