I usually use a graphite pencil, but the site below also lists a waterproof pen.

Here is a link that might be useful: markers

John, I agree that many of the self-labeled permanent markers are anything but. I've had several nice labels that faded to "spots" of ink just in the locations where I started and stopped each letter--totally unreadable after awhile. Like Ann, I've switched to graphite pencils for most of my outdoor plant labels. I just buy a soft one from an art or stationary store (okay, okay, I've even been known to use a golf pencil when I can't find my "usual" pencil).

Let us know if you find something better. Ann's link looks very interesting. Has anyone in the group tried them??

Take care,
Grant

The concensus in my daylily club is also that graphite pencils last longer than permanent markers, even years. I use them on the little plastic stakes and they remain legible.

Another thing that works is to buy waterproof labels and print them on a lazerwriter, then apply the label to the venetian blind slat or whatever. The waterproof labels come in clear and white. I use the white ones to recycle metal plant markers by putting them over the first label.

Some people also like the plastic labelling tape that is typed on by a handheld machine. My mind is a blank what you call them.

I was given some aluminumn ones about 20 years ago. A pen would make and impression of letters into the soft label. Truly as permanent as the aluminumn tag.

I use plastic tee stakes. See the link. I write on them with a pencil. I keep finding my old ones buried in the garden; they are still readable after 18 years. The plastic will last around 8-10 years if exposed to full sun.

Here is a link that might be useful: plastic tee labels

One thing to keep in mind if you're using these vinyl blind slats in a vegetable garden is that they are shedding lead into your soil and onto your hands every time you touch them. I'd also be leary of using them anywhere small children might be able to get their hands on them.
That said, the most durable labeling I've found yet is to use one of those hand-held label printers (~$30 or $40 at an office supply store) and stick the labels onto metal or plastic stakes. I have some in my garden that have gone through 2 winters now, with no deterioration.

Thanks all for your input - I shall start using a graphite marker now. Mystery solved!
John

One time I had a Pentel permanent fine tipped marker. The ink lasted for years without fading. It ran out of ink and I've never found another one like it.

Now I always use those "peel off china markers". I've been using them for 4 years now and so far the writing has always stayed on looking just as good as when I first wrote it. The writing is a lot darker and clearer than pencil, but it is a bit hard to write in small letters.

If you want to write a heavy, thick line with a pencil, go to the stationery store and buy some drafting pencils with the soft "lead" in them. These are cheap wood pencils, nothing fancy. They are hard to sharpen though. The lead is so soft that it keeps breaking. The best way to sharpen them is with a file or a piece of sandpaper. I gave up on them; I just use a mechanical pencil.