While the usage of ought to has been declining steadily in the last 300 years, it is still incredibly common. (The linked Ngram shows that it is roughly 1/3 as frequent as the word table today, and table is not at all an obscure word.) The question is when should you use it, and when to use should or must. Why should I use "ought to"?

Understanding the Context

- English Language & Usage Stack Exchange Is it appropriate to omit to after ought? I ought to be disciplined for my insolence. Vs. I ought be disciplined for my insolence.

Key Insights

Is it okay to omit the to? modal verbs - Is it appropriate to omit "to" after "ought"? - English ... As a result of this, the following "rule" is valid for remembering the modern English spelling of past participles that end in -aught/-ought: the spelling -aught contains the letter "a," and is used in the past participles of verbs that contain the letter "a" in the present tense (teach and catch) while -ought is used in the past participles of ... Spelling with 'ought' or 'aught' - English Language & Usage Stack Exchange 5 It's usually ought not to.

Final Thoughts

The Google Ngram below shows that most of the time ought not is used, it is the first two words of ought not to. Many of the remaining cases appear to be constructions like ought not publicly to, with an adverb between the not and the to. Verbs like ought, should, must, have to express degrees of desirability or probability, neither of which really apply to non-sentient things like branches. Maybe the branches "ought not reach" according to the writer's preconceived notions of whether they were likely to. Or maybe it's a type of plant that can extract soil-based nutrients from far-reaching branches (like weeping willows, I ...