I actually got farming to the level I did because I became addicted while getting to "the minimum level to accomplish all quests" Of course I dislike quests and only do them when I want something opened up to me but didn't want to be restricted by a level requirement later. So learned the basics and got hooked by the fact I had the seeds and hate wasting xp. Being that my favorite skill is herblore it was easy for Jagex to reel me in.
That being said I only do trees with any seriousness getting all my seeds from the kingdom or the Mole. I have no real level goal for farming I'm just compulsive in doing my runs once a day normally. With this as a background let me reply to a couple of your suggestions.
All plants are different and flowers are notoriously touchy. Calquats die very frequently at lower levels and very rarely in the 90s; poison ivy never ever dies, no compost and payments required. Evil turnip is indestructible, although I just planted a few of them. I find that this makes farming unique and much more interesting than cooking lol.
I feel level should have more of an effect on death rate but if not at least let the compost or no (super) compost have a noticable effect I get more benefit from selling my excess super C than I do in actually using it. I will also agree that the Evil Turnip seems to be death proff.
If you are harvesting corn and melons, I would use scarecrows/flowers to protect them, but this is obviously your call. Watermelons are great xp and cost quite a bit; I would definitely protect them with flowers.
I normally protect my melons but stopped protecting my corn when I started growing marigolds and limproots and out of the principle that at my level and the level required to grow corn "should" mean they survive at a better rate then they do.
You will actually save money if you do not protect yews; their death rate is 1 in 7, just take a look at the cost of payments vs. extra seeds you can buy instead. The death rate for palms is 1 in 9, but papayas are much cheaper now than in the past, although they’ve been going up recently. I don’t have the numbers for mages.
In regards to Yews the only reason I have a cactus is to get spines to protect my Yews so for me it is a wash. Like I said since I have no level goal I have no real reason to buy seeds other than maybe a corn or melon seed to round out a set of three. I normally only buy an item needed to protect a tree, but I have been saving targeted fruits up for a while so normally I have the payment already in the bank. I just planted my first Palm and found that I don't have nearly enough papayas to not have to eventually resort to the GE.
Overall I know I'm not real effeicient in my farming but it is one of the few skills I consistently make a profit at and with my work schedule it requires less of a time commitment than many other skills to get a similar benefit. Farming is as I mentioned a rather addictive skill.
