I would definitely deadhead the penstemons and the salvias. I know some people leave the salvia heads on to overwinter but if you cut them back now you may well get some (smaller) flowers. You can sometimes spot tiny flower buds in the leaf joints. Are the wallflowers the perennial or biennial type ? It looks as though they've already gone to seed and very woody at the base so personally l would pull them out. If they're the perennial type you could try taking a few cuttings before disposing of them.
Petunias usually "deadhead" themselves although you can pull them off (they're very sticky), but it does mean that you end up with the latest flowers at the end of long stems. I hedge my bets and cut about 2 thirds of them back and then give them a feed.
It makes the plant more visually appealing, why you planted it. Secondly, if the plant is a perennial or has a long flowering season, it stops it using energy on unwanted seeding and encourages the production of more flowers, either this year or next.
Many plants often stop flowering once they have set seed, as they have fulfilled their biological purpose by producing the next generation.
If you are not wanting to save any seed produced, then dead heading is probably desirable, but not essential, though the floral display may suffer
I think the perennial wallflowers run out of steam eventually, Bowles Mauve is a prime example. My next door neighbour has kept biennial wallflowers going for 3 or 4 years but they get very leggy . The weather probably does have a bearing, but l couldn't say for sure
As others have said dead heading can increase more flowers during the season. However other plants being left to seed will provide seeds for the birds later on in the year.
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Are the wallflowers the perennial or biennial type ? It looks as though they've already gone to seed and very woody at the base so personally l would pull them out.
If they're the perennial type you could try taking a few cuttings before disposing of them.
Petunias usually "deadhead" themselves although you can pull them off (they're very sticky), but it does mean that you end up with the latest flowers at the end of long stems. I hedge my bets and cut about 2 thirds of them back and then give them a feed.
My next door neighbour has kept biennial wallflowers going for 3 or 4 years but they get very leggy . The weather probably does have a bearing, but l couldn't say for sure