I have a couple boxes of Miracle Gro in my cabinet (the kind you use a few tablespoons of per gallon water). They’ve been in the cabinet for 3 years.
I also have a small bag of flower fertilizer. Says it’s for roses and what not. It’s been in the carport for at least 2 or maybe 3 years. Bag has been opened and used, and it is sealed with a zip lock type seal.
This stuff is pretty expensive. Does it lose strength or potency with time?
I hate wasting money. But, I also don’t want to use this stuff and not get good results with my flowers.
A cite would be wonderful. But personal experience is welcome too. 
The fertilizer’s fine. The stuff will keep indefinitely if it’s dry, and only lose any nutrients if it gets wet and runs out of the bag.
Cite: the law of conservation of matter
I wasn’t sure if fertilizer was affected by time like medicine.
I found out the hard way Bactrim (sulfa drug antibiotic) loses potency. I had a urinary tract infection and my urologist started me on Bactrim while he waited for the urine cultural results. I was still very sick with fever when he called four days later to tell me the culture showed Bactrim would kill it. We were both confused until I looked at the bottle I had and it was dated 5 years earlier. It was some I had left from a hospital visit for a kidney stone and they had sent me home with a script then. I quickly took the script to the pharmacy and was feeling pretty good within 48 hours.
I used 5 year old Miracle-Gro as a base for hydroponics solution last year when I restarted my rig and the vegetables grew just fine. As that would be just about the only source of nutrients for said plants, I’d say that’s proof the potency of “bagged fertilizer” doesn’t drop off when stored and kept dry.
That’s good news then.
Fertilizer is so darn expensive. Especially the special mixes for azaleas and roses. I didn’t want to throw this stuff out unless necessary.
I’m planting a few flowers today and plan to use the granule stuff for roses.
I like Miracle Gro to for most things. I did find out the hard way that it can’t be sprayed on Crape Myrtle bushes. They all got these ugly spots on the leaves a few days later. I had to wait until Fall when they dropped their leaves before the problem went away. Thankfully it didn’t effect the flowering in June and July.
These days I use the Miracle Gro pre-mix bottle stuff and the sprayer that comes with it. I “do” still need to go old school and use up the powered stuff I still have. That means mixing it in a watering can and hand pouring.
Considering that the ingredients in Miracle Gro are mostly basic elemental compounds, I doubt that any loss of nutrients from degradation would occur even if it got damp; phosphorus, potassium and the like are elements (or simple compounds) and thus cannot degrade in any way (unlike, say, sugars, protein and fats found in human food), with the possible exception of nitrogen, which can escape as gas if any degradation occurs.
Also from experience: damp/wet Miracle-Gro congeals into something like concrete. It’s not that it degrades, it’s that you have to pulverize it before you can mix it into solution and use it. Much easier to simply not get it wet.