Lori, that's a very valid issue that anyone who gardens over a period of time runs into. I didn't start getting actually 'serious' about keeping records until the '90s ... because, hey - we always think, 'well, I'm going to remember THAT...' - right up until you don't / can't remember.
And even if you do / can remember a specific plant that you got from your grandmother / aunt / cousin / neighbor / someone you worked with 10 years ago / for a birthday / Christmas / just because...
???
Well, really - after a while: you won't.
I have used garden journals .. yeah, written - then that got too hard to locate a specific plant, so I upgraded to digital ... still have over 20 years' worth of digital garden journals (Word program / .doc files). And that gets too complicated, cause you still have to remember (approximately) what year did that happen...???
- But at least a digital file is searchable / can be scanned for key words and will keep track of the pictures if you add them.
And I started a digital file of 'ALL' the trees, shrubs, perennials, bulbs, herbs (etc.) in Excel ... columns of data on every plant in the gardens. Not bad, till you forget to add one...
I also have two more digital Excel files for my seed inventories - one for edibles (food crops for my kitchen garden, I have more than 800 seed varieties in inventory) and another just for perennial flower seeds.
And not for nothing, this is the digital age so I have been gradually adding a total list of 'all' to my profile on garden.org (feel free to look) for quick reference.
I also keep specific daylily log books (alphabetical) in a printed out disc-bound notebook system, complete details on every daylily and all my DL seedlings / breeding program, because: 1) you can add / remove pages or change them around to different locations easily; 2) I don't have to turn on a computer to look something up; and 3) I can physically carry those out into the gardens to compare to the actual plants and add notes / update info all while I am standing right in the gardens.
And I keep a second disc-bound notebook system on my Iris ensata seedling project as well, these are all 100% unique and one-of-a-kind individual plants that, some day looking back, I may be very glad I kept good records for.
And I keep a third disc-bound system for my annual planting plans for all the food crops, herbs and companion plantings for my kitchen garden... keeping records from one year to the next on everything from how many and which heirloom tomatoes I grew in each year to how many and which varieties of potatoes I planted vs. pounds yield and what factors (such as weather) may have influenced the outcomes.
I've also found various printing systems and software programs may be useful (one for printing garden tags that last longer in zone 3 weather) - and have been working to tag every single tree, shrub, perennial, etc. in the gardens with permanent metal tags ... still not done with that, but I now automatically put a metal marker on every single plant as it goes into the garden.
So, final answer? There isn't one. Try several ways - online or digital are great, but keep a full off-line backup that you can get to or look at when the computer has crashed or you just need a quick reference.