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How big are the Yuccas and are they inside or outside? What do you mean by 'started well in growing'?
Is there a reason why have you not watered since November?

Firstly, they won't survive the winter outside over here. If it's inside, and I'm assuming it is. They need a sunny position and very little water. Let the soil dry out before you water it again. You could also try misting the leaves for humidity.

Hi, welcome to GoY, can you tell us where you have these plants, as Myron says they will not survive a british winter, they need a minimum winter temperature of 50 deg f, but bear in mind that in their native habitat they can reach a height of 30ft with a spread of 15 to 25 ft, although, realistically they will never get to this size in the UK. Derek.

Indoors, I would expect to water them about every 10-14 days, depending on temps and humidity. Yucca elephantipes is a subtropical Yucca, and needs more water than the desert forms--which make lousy houseplants, anyway.