Even in places where they thrive it is good practice to cut them back hard in spring every few years. They are very ugly twiggy bushes and look much better with regular rejuvenation pruning. It might be useful when trying to keep them happy to think about where they come from. Their natural habitat is in China where they grow in forest clearings, riverbanks and limestone outcrops. So they prefer lean, fast draining soil but with plenty of moisture at the root. They are a woody weed here and are particularly prevalent along railway tracks (they like the ballast), quarries, urban wasteland and disturbed areas. I agree with Ken about the butterflies. Although Buddleja attracts adults to the nectar they are not a food plant for butterfly larvae and so they do not provide a good all round butterfly habitat.

Yeah, of course. Not sure how I missed that, as I'm also growing Black Lace (although not very successfully between the deer and probably verticillium wilt)

Not a plant I know from personal experience but compare with Baccharis halimifolia.

" in spite of showing the most common, everyday symptom of lack of nitrogen: general yellowing. " And this may be where some of the confusion is coming in. A nitrogen deficiency is NOT characterized by a "general yellowing".....at least in an established or older plant (may show as a generalized off color in seedlings). It is the oldest (lower or inner) growth that first shows symptoms of yellowed foliage, never the new growth. That's where the plant pushes all of its nutrient reserves. If this were the issue with the plant in question - lack of adequate N - then we would see internal discoloration, not the external coloring the photos illustrate. And the new growth would not be as lush or profuse or as dense as is shown. And that is simply the way that specific nutrient deficiency works....it is not really up for debate! Yard, I am really having trouble understanding why you are not willing to accept this, including the input from two quite experienced horticulturists/plantspersons (floral and Embothrium), leaving my comments aside. It's not like yews of all types are rare commodities in either the PNW or the UK and both individuals confirm that very chartreuse, acid yellow/lime green is not an uncommon coloring for any new growth yet you seem to dismiss that as inconsequential or limited to only selected cultivars. I wish I had taken photos of the new stock at my local nursery when I was there last week, as ALL the yews were showing similarly colored new growth. It only lasts a matter of a couple of weeks before it takes on the more typical darker green color. And there is also the very real consideration that color perception is largely subjective - what seems anemic or bleached out to you looks perfectly normal to others - and that photos seldom portray color with total accuracy. I have to say that I do not see any "yellow" in the OP's photo.....just a very vibrant lime green. And the shrub appears to be extremely healthy with dense new growth and no obvious need for any fertilization at this time.