I'd think the sponges would be a bad idea. Sponges, kept constantly wet and contaminated with organic material in your kitchen, go sour fairly quickly. That's why I put mine in the dishwasher each time it's run. If you wish to increase the water holding capacity, don't add pumice to the Miracle Gro Moisture Control Potting soil (pumice increases drainage, making the water flow out FASTER), just using the soil out of the bag. Or, add some of the water saving crystals which absorb water and allow the roots to grow into it to remove the water from it.

Thank you roseseek. If I use sponges, it would be new from the store, I would not use the used ones, :)

But you think it is still a bad idea? So roses will be ok with water saving crystals? We have not had any rain since February(I think), So just pure Miracle moisture control potting mix in grow bags? I thought I read somewhere that recommend 50-50 with perlite,

No, I wouldn't use sponges period. The soil has organic material in it and that dissolves in the water and absorbs into the sponges. They won't air out and will hold that organic contaminated water. I honestly fear their souring, but that is up to you. I don't know where you may have read to mix the Moisture Control Soil with anything. Perhaps if you lived where there was regular rain and the fear was the pots would remain too wet. I don't know where you live nor what climate type you are in, but in Zones 9b and 10a, Los Angeles County, Santa Clarita Valley, San Fernando Valley, Pacific Palisades (right on the water) and Santa Maria in northern Santa Barbara County, the MG Moisture Control Potting Soil straight from the bag in pots ranging from a gallon to more than fifteen gallons, and in hanging baskets, has worked just fine. All of those areas are subject to periods of very low humidity, extreme aridity, breezes to horrific winds and mild to bellows of Hades heat. ALL of my roses, including the seedlings, are planted in pure MG Moisture Control Potting Soil. It's ALL I use as it takes the longest to break down in my experience and does appear to help with watering. It doesn't break down into "muck", sludge, so many others do. It remains granular, almost like coffee grounds, and I water COPIOUSLY by hand. I even have citrus and apple trees planted in it and it works as well as it does for the roses. So, if your conditions sound comparable to what I have described, I think you can feel safe using it straight. I wouldn't use the moisture crystals with the Moisture Control soil. I would think it shouldn't be needed. But, definitely get rid of the pumice and don't use more perlite. You want it to hold water, not drain quickly, and the soil drains just fine by itself. I honestly have used it since they introduced it.

Thank you roseseek! I think sometimes reading so many things on internet I got mixed recommendations and confused, I will take your advice and will try growing a few roses in grow bags this fall and see, if it dries out too fast, I can switch to regular pots before summer.

You're welcome! Really, no matter what you read, you will never know how it works where you are until you DO it. Use the bags and keep the soil straight from the bag. Those grow bags will dry out faster than nursery cans, so you want all soil and nothing to add to the drainage. Have fun!

NO, to sponges! I don't put anything in my pots that holds water in the bottoms or that will withhold water from the roots. I have tried the water crystals but didn't care for them either. They turn into a gooey jelly mess in the soil. I also stay away from terracotta pots because they wick water out of the soil and evaporate it before the rose has a chance to use it.

My only question about using the grow bags for climbing or tall roses, like DAs, is if they would be stable enough to keep them upright in any kind of wind? Tall, fully leafed out roses can act like a sail in the wind and I have had large, heavy steel rose pots tip over in a Michigan gale.

@seil zone 6b MI thank you for your comment. I saw people posted that they grew small fruit trees in these grow bags (15-20 gallon bags). In our area, we do get strong wind too. Just this week,one of my patio chairs ( rot iron ) was brew over several feet away. I was thinking about tied the handle of one bag to another like a chain and each end tie it to something heavy, Not sure how that would work or handles might tear apart?