Which script should my character use when writing in druidic?

Question

Which script should my character use when writing in druidic?

Druidic (PHB 66)

You know Druidic, the secret language of druids. You can speak the language and use it to leave hidden messages. You and others who know this language automatically spot such a message. Others spot the message’s presence with a successful DC 15 Wisdom (Perception) check but can’t decipher it without magic.

Druidic is also not listed in the languages tables in the PHB 123.

I am looking for a RAW answer (maybe from previous editions, if it's not specified in 5e), as many languages are specified in the rules to use a specific script (like in 3.5e, Giantish was said to use the Dwarven script and kobold to use Draconic). Failing that, lore from an officially published TSR/WotC D&D product, including novels, that describes Druidic script would be helpful.

Answer 1

Some languages use symbols rather than letters and scripts.

The rules don't specify a particular script or family of runes, which leaves the detail of how the secret Druidic language is presented in written form up to each DM/campaign.

The PHB text that you cited is what you have to work with in 5e, RAW. So what do you do? Tapping into previous editions is one choice.

An example of previous edition lore: AD&D 2e

Script per se may not figure into the druidic language at all. For example, in AD&D 2e, in the Complete Druid Handbook, under the heading "The Secret Language" we find:

Finally, the secret language of the druids remains a purely spoken tongue. A few simple runes or marks (symbolizing danger, safe water, safe trail, and so on) exist for marking paths and leaving messages, but the language cannot communicate actual sentences and complex ideas in writing

This points to the Druidic language as being symbol based, not script based. That has the potential of making Druid Scrolls problematic (as they were in 2e). This can be an obstacle if you want Druid spells to be available on scrolls in your 5e campaign. It might also be workable, in that the Druidic language and Druidic magic are rooted in the same basic philosophy and symbolic system. When it comes to magic, symbols often substitute for words/letters, as shown in the Glyphs of Warding in Clerical magic, for example. Druid symbology written onto scrolls can get around the obstacle and make spell scrolls viable.

The Rules Don't Try to Cover Every Granular Detail.

Where the rules are sparse, the DM and the players are expected to flesh out the detail in a 5e campaign. That's a feature, not a bug.

Recommendation: treat the Druidic secret language as symbol based, rather than script or letter based, using the guidelines in AD&D 2e as a point of departure.

Aside: historically, a symbol-based form of writing, hieroglyphics, was very effective for the Ancient Egyptians.

Answer 2

Which script should my character use when writing in druidic?

If this is when they are using it to pass messages then consider that there is no script but the message is passed in another form, maybe something like trail signs, i.e. knots in tufts of grass, leaves folded in certain ways or stones and berries placed specifically. A druid would leave messages to be picked up by others.

If you want the messages to be passed along rather than left to be found then they could be passed as images of these signs, so a doodle in the corner of a misleading message written in the characters typical style contains the druidic message, or in a book of plant illustrations the images of the plants hold the druidic message. Similar to answers to this question https://worldbuilding.stackexchange.com/questions/18736/encrypting-maps