Keep the fern in the same pots they come in, every other day submerge them in a 5 gallon bucket filled with 1/2 cup of epson salts amp; 3 gallons of regular water until the soil stops bubbling, then hang up to drip dry... ferns will be dark green, glossy, and 3x3 by September from ferns that start out with 7 fronds in May.
How to grow HUGE Boston Ferns: 1) replant in 12-14 in diameter pots; 2) re-pot using a mix of potting soil and some horse manure; 3) water frequently (2x/day in hot weather); and 4) if they start losing their dark green color, find some good fertilizer for acid loving plants and follow the instructions.
Instead of using commercial fertilizers and plant food, why not use some scraps from your kitchen that would otherwise end up in the trash or compost bin to amend and improve your garden soil naturally? Inexpensive and easy, these are my favorite ways to get my garden ready for spring planting. Eggshells, banana peels, Epsom salts, coffee grounds. Find out more at http://www.fresheggsdaily.com/2016/06/7-ways-to-improve-your-garden-soil.html