fertilizing the garden


Hello, I have read your information about soil. My question though is this. Every spring I put down manure around my shrubs and perenials. I would also like to give my flowering shrubs and perennials a boost during the growing season. especially my Azalea, Peony, Trout Lily, Solomons Seal.The garden centre sells the Parkwood Flower fertilizer (15:30:15) and Miracle Gro (24: 8: 18)
Please could you tell me the best way to feed these plants? My soil is in good shape as I amend it when needed with compost. It seems difficult to test the soil regularly.
What do you recommend? And how often.
I also have an azalea and they like acid soil. How to feed that one or do I buy the azalea food fertilizer from the garden centre?
Another question is this. When I bought my Lilac shrub a year ago, it was a deep purple, now this spring it is very pale. The same goes for the colours of the purple coneflower. Totally different colour to when I bought them. Is this related to feeding?
Many thanks
Ruth Oppenheim


Thank you for contacting the Toronto Master Gardeners about fertilizing the plants in your garden. The best way to feed your plants is to feed your soil which will in turn feed your plants. It sounds like you are already doing a great job at this with your annual applications of manure and  compost as needed. I’m not sure what you mean by as needed ? It’s a good idea to apply organic matter (manure and compost are both types of organic matter) to the surface of your soil every year. I’m also not sure why you want to ‘provide a boost’ to some of your plants during the growing season – are they showing signs of nutrient deficiency ?

A soil test is the best way to to get information about the health of your soil so they you can make any amendment to it as needed. This should be done every few years. If you haven’t done one or not for a while, I highly recommend it so you will know where things stand. If excess nutrients are added to soil, they will leach into the ground water and eventually into the water supply. Basic soil testing kits are readily available at big box stores. You can also have this testing done by a lab, that usually provides specific recommendations about what amendments might be needed. However you choose to do soil testing, I suggest that this include testing of your soil pH (this is often part of soil testing). This is the one area where I think your soil might need some help. Some of your plants (especially your azalea) prefer acidic soil / lower pH than most plants which prefer soil pH between 6 and 7. But you should test for this before trying to change the soil pH. There is more information about soil testing including labs in Ontario where this can be done and about soil and how to keep it healthy here in Soil Fertility: A Toronto Master Gardeners Guide.

Specifically about the plants you are considering for ‘a boost’ :

Azaleas (Rhododendron spp.) typically don’t require much fertilizer if they are kept well mulched (they will be fed as organic mulch breaks down). However they do require acidic soil (pH between 4.5 and 6) in order to flourish. Once you know what the pH is in your soil, you can apply ferrous sulphate around the base of the plant to lower the pH, or apply ground dolomitic limestone to raise the pH. There is lots of good information about caring for azaleas at this link for the Azalea Society of America.

Peonies (Paeonia lactiflora) that are established and have been growing well only need to be fertilized every few years. If they are over fertilized (especially with too much nitrogen), they could produce fewer blooms. You can find more information about caring for peonies at this link for The American Peony Society

Your trout lily (Erythronium americanum) is a spring ephemeral. It blooms in mid-late spring and then goes dormant. It does not need extra fertilizer if it is planted in rich, humusy soil. The best way to feed it is to apply organic matter to the surface of the soil around it each spring before the new growth starts. This is a slow growing plant that might take up to five years to bloom. It prefers acidic soil. You can read more about trout lily here

The best way to feed Solomon’s seal (Polygonatum spp.) is to allow fallen leaves to stay on the soil surface around it. They will feed the plant as they decompose. Alternatively you could apply organic mulch. Here is a great article with lots of information about Solomon’s seal.

Regarding your lilac (Syringa vulgaris) and its flowers that are very pale compared to last year, these flowers do fade as the plant ages, but you have only had your lilac for a year. I think it is most likely that this has happened because of the temperature fluctuations including early and intense heat that we had for a short while earlier this spring when the flowers were developing. Fertilizing lilacs within two years of transplanting can damage the roots. Typically they do not require much fertilizing.

The colour change in your coneflowers (Echinacea spp. and cvs.) is not at all unusual. Most coneflowers are hybrids, and over time they do tend to revert to the colour of their parent species. Cross pollination of different coneflower varieties can also result in colour change.

About the fertilizers available at your garden centre, 15:30:15 is typical for a fertilizer for flowering plants (the middle # is phosphorus which promotes blooming). 24:8:18 sounds like it is intended to promote the growth of foliage. The first # is nitrogen, and fertilizers high in nitrogen often promote leafy growth at the expense of flowers.

Further reading about fertilizers and soil can be found at these links :