Hello Margaret Yellowing of box in planters is often to do with the soil running out of goodness. If you can repot them in fresh soil then they will be much happier. Try and get rid of as much of the old soil as possible - you can wash it from the roots if you wish.
I have a life size pig at our caravan I wish it was at home which I have two large pots with bay trees in,I have a yew which I am still wandering what to do with its about 4' high 2'wide open to surgestions.Ive have done topiary on hebies in the past they grow quick and look nice when they flower.
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Yellowing of box in planters is often to do with the soil running out of goodness. If you can repot them in fresh soil then they will be much happier.
Try and get rid of as much of the old soil as possible - you can wash it from the roots if you wish.