Hi Annadell,

Thank you for your question regarding yellowing Maples. Maples are difficult to raise here in Colorado and there are certain needs that must be met.  

What is important to know is the make-up of your soil chemistry. Generally, Maples need acidic soil and Colorado has mainly alkaline soil. Because this is happening in your front and back garden I would suggest doing a soil test before applying any more iron. 

You can do this by contacting:
https//agsci.colostate.edu/soiltestinglab/

• They will give you the information how to gather your sample and where to send it.

I also will give you information regarding Chlorosis and Iron amendments, here's a quick look. 

• add iron sulfate to soil – effectiveness may be limited
• add iron chelates to soil – some are more effective than others depending on soil alkalinity
• addition of sulfur to lower soil pH – less feasible when alkaline soils are high in lime
• iron sprays to foliage – often impractical, especially on large trees, effect is only temporary
• trunk injections – holes drilled in trunk may provide entry for disease organisms; best left to certified arborists

For more info go to:
Iron chlorosis in trees:
2121 – Iron Chlorosis in Trees – PlantTalk Colorado (colostate.edu)
Maple Trees:
1733 – Maple Trees – PlantTalk Colorado (colostate.edu)

Please let us know if you have any more questions:

Lisa