That's a two edged question. You don't want to do anything to stimulate new, soft, sappy growth until the chance for your last frost is probably past. Why waste the effort and money only to have the plants frost damaged due to growth much more susceptible to damage than harder wood? Then, organic fertilizers require 'digestion' by soil bacteria and fungi to release their nutrients to the plants. Most of these are dormant in cold soil so the prime benefits of your expensive food is often wasted. You would be far better off (as will your roses) to wait until the air temperatures begin to warm closer to the seventy degree mark, so the soil can begin to warm and stimulate the bacteria to begin "eating" the organics. Remember that colder night temps cool the soil back down, slowing the bacterial action and reducing the benefits of the fertilizers. If your plants are potted, they'll begin to be able to use the fertilizers earlier because sun shining directly on the pots heat them up faster, earlier, so there is more digestion going on while they're heated up, but they also cool off faster as that absorbed heat radiates back out into the colder night air. The actual date is going to vary greatly by your micro climate, but you can figure that out based on your air temps and when you think it's safe to trigger the soft new growth without subjecting it to certain freezes.

First of all, according to various people, you may want to reconsider keeping your catfish and pond fish together. I posted a link below - or you can do a search yourself on the subject. In our pond in Upstate NY (similar in zone to you), I remove all my tropical plants in early October. We cut back our hardy water lilies which are planted directly in the pond and drop to the bottom the other hardy plants. Our pond is 3 feet deep and approx. 3,000 gallons so the fish overwinter in the pond. I don't know what you should do with a pond that small. We purchased a plastic 100 gallon tub at Country Max for our tropical plants, and I think that would work well for your fish if you decide to bring them indoors. You would have to add a pump or air stone for aeration and something for filtration. My husband altered the "plug" with a faucet and we can drain it pretty easily with a garden hose into our basement sewer come spring. Here is a link that might be useful: catfish in a pond

I'm no expert but I believe warm temps are key for these plants. Once it is warm enough they will start putting out new leaves and at that point I believe you can begin with the fertilizer. (That's when I start mine) If its too cold the plants don't do much. They just kind of sit there dormant. I don't think fertilizing in cool weather will do much.

The "experts" growing for resale are almost always growing in pots, which is like talking about apples and oranges. Pots do not have the features of the soil and probably do need to be fed. Even those growing in the soil have moved the soil about and broken down the natural process and they will only be growing in the soil for 1-3 years so adding fertilizer could be called for. Don't take this to say fertilize the soil for a permanent planting because the sooner the soil returns to a purely natural process the better and fertilizer can and often does interfere with the process. If you plant a hosta and plan to leave it then all you need is organic matter. 20-20-20, 17-17-17, 10-10-10 represent the percentage of the NPK (only) by weight and the cheapest way to get NPK is the best. A 40lb bag of 10-10-10 has 10% NPK of 40 lbs while 20-20-20 has 20% so the question what is the cost per pound each of NPK. Now, 10-10-10 has 10% NPK plus an inert "carrier" so you may pay the shipping and bagging cost of the inert carrier. Back to figuring out the cost per pound of the NPK not the bag cost. One of the inert carriers is limestone, which may not be good for your particular soil. Back to question of which bag to buy, the cheapest way to get a pound of NPK is the best such as buying high concentrations like 30-30-30 urea and whatever the highest concentration of phosphorus, etc and make your own mix. DonÂt waste your money on trace elements which may be needed in a pot but mulch will deliver trace elements to the soil. Is this more than you wanted to know, or do you know the time and donÂt really want me to build you a watch?