"Buderschnookie" expounded:

They need food.
If they yellow from the bottom up they need macronutrients, and if they
yellow from the top down it's micros.

No, even sunflowers planted in fertile soil show browning of the lower
leaves, it seems to be the way they grow. I just pull them off.
--
Ann, gardening in Zone 6a
South of Boston, Massachusetts
e-mail address is not checked
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