Hi, Ada!
Thank you so much for reaching out to UNH Extension regarding your Fall fertilizing question.

Generally it is not recommended to fertilize plants after the first hard frost, which most of NH has already seen this season. At this point, most plants have gone dormant for winter. That said, you can absolutely take the time now to mulch your plants to get them ready for cold conditions. I personally like to use shredded leaves in my beds, however they can be a bit pesky to keep in place, especially on windy days like today. Here is a link to some other mulch ideas: https://yardandgarden.extension.iastate.edu/how-to/how-overwinter-plants

Your plants might benefit from an application of fertilizer in the spring (see here for a very thorough article: https://extension.unh.edu/resource/fertilizing-trees-and-shrubs-fact-sheet). A soil test through UNH Extension could help identify if any of the key or trace nutrients in your soil are deficient that might require a specific type of fertilizer. Here is information on how to submit a sample for analysis: https://extension.unh.edu/agriculture-gardens/pest-disease-growing-tools/soil-testing-services

If you feel that your plants need a boost in root production next Fall, your best bet would be to apply a potassium-based fertilizer, such as potash (so long as the soil testing results don't show an overabundance already in your soil) would help with root development. It would be best to apply in September/October to give your plants the best chance of soaking in the nutrients prior to going dormant for winter.

Take care and have a great gardening off-season! -Michelle