Q.Lemon tree and gardenias
My lemon tree leaves are turning yellow. What does it need? My newly planted gardenias are whithering away. My husband had me put cow manure around them because he said they would thrive. However, they were shocked! I tried to wash it all away but it still looks awful. Can I save them?

Yellowing leaves can indicate watering issues; too much or too little.
You should also check your plants for signs of pest or disease.
These articles will help you pinpoint the issues.
https://www.gardeningknowhow.com/edible/fruits/lemons/common-lemon-tree-diseases.htm
https://www.gardeningknowhow.com/edible/fruits/lemons/how-to-grow-a-lemon-tree.htm
https://www.gardeningknowhow.com/edible/fruits/citrus/tip-on-water-requirements-for-citrus-trees.htm
Hot cow manure yes, was likely way to much nitrogen for your plants.
Remove as much as possible and flush the soil.
Manure should be properly composted and then is a great soil amendment for most plants.
Gardenia are finicky plants and may not recover.
This article will refresh you on the care requirements.
https://www.gardeningknowhow.com/ornamental/flowers/gardenia/gardenia-plant.htm