water

Are all these correct forms?

There are plants to water.
There are plants to be watered.
There are plants (that) need watering.

In standard English, 'that' is required not optional in the third sentence because it's the subject of the clause 'that need watering'.

What David wrote otherwise yes.

<In standard English, 'that' is required not optional in the third sentence because it's the subject of the clause 'that need watering'. >

You mean, in written an/or formal standard English, right? Informal and/or spoken standard English allow omission of "that".

I have never heard of a variety of English that allows "There are plants need watering." It's always "There are plants THAT need watering."

"You mean, in written an/or formal standard English, right? Informal and/or spoken standard English allow omission of "that"."

Omission of 'that' is allowed it's the object of the clause not the subject'

There are plants (that) I need to water

Non standard varieties allow omission or even the use of 'what' as a relative pronoun.

There are plants what need watering

''There are plants to water.
There are plants to be watered. ''

both are correct, just like

Rooms to let.
Rooms to be let.

Most Americans say :

Water those plants
Those plants need to be watered
Those plants are dry
Those plants are dieing

Guest, that'll be "dying"

those people is dont need a water because she/he hate water hehehehhehehehheheh