Lasagna mulching is very effective against seedlings, including the snapdragons. It will be mediocre-to-good in preventing the corms from sprouting. The key will be in making the newspaper layer very thick to prevent light from hitting the soil, and in smothering any little sprouts that do try to poke through it.

I'd suggest at least ¼" (½" is better) thickness of sopping wet newspaper, overlapping the sections about 2". Top with 2" to 3" of organic mulch such as shredded leaves. To plant, stab with a sharpened trowel and put the transplant into soil, then mulch around each transplant. Keep a close eye on them and pull up any glad sprout that tries to push the transplant out of the way. I do not recommend direct seeding this year or next year -- instead, start the seeds in pots and then transplant into the mulched bed.

Remember that the newspaper must be completely concealed under a thick layer of mulch, and to keep the mulch moist. Drip irrigation is superb for maintaining the proper moisture level.

Unless you sift the soil to remove all the corms, it is likely that you will have those corms attempting to sprout for several years. Because DH shoves the mulch aside so he can direct-seed, I am still removing cormsprouts from a bed that hasn't had glads in the past five years.

Good luck and happy growing!

Does anyone lift glad corms every year?
How often do you thin the corms?

How long after blooming do you dig them?

Thanks