Hello, what is wrong with these Rugosas? The same thing happened last summer, I added composted manure around them, and sprayed an organic foliar fertilizer, it did not help with yellowing of leaves. They still bloomed and produced huge rose hips. They are on a South facing slope in somewhat sandy soil. They are mulched.
There is also some browning of the branches. Thank-you.
One of the common problems Rugosa roses can face is interveinal chlorosis, which is a deficiency of iron in the soil which shows up in the plant’s leaves. This is often apparent on younger leaves first, as your photo shows – they may be pale green, yellow, or even almost white in the most severe cases, with darker green veins. Here is a link to a description of chlorosis, its causes and some methods for ameliorating it: https://www.missouribotanicalgarden.org/gardens-gardening/your-garden/help-for-the-home-gardener/advice-tips-resources/pests-and-problems/environmental/iron-chlorosis.aspx
You have already used a foliar spray, which will help – but only in the short term – and you have already mulched, so you are doing the right things. It is worth testing the pH of your soil. If your soil’s pH is above 6.5, iron may be bound up in your soil and unavailable for your plants to use. Most large home and garden centres sell devices for measuring soil pH. If your soil has a higher than desirable pH level, you can apply an iron fertilizer, making sure that you follow the directions carefully.
Agriculture Canada notes the complexity of iron chlorosis in trees and shrubs and recommends the following treatments, noting that although they are not always successful, there are several to choose from:
- Foliar sprays: At the onset of symptoms spray leaves with a ferrous sulphate solution (28 grams or 1 oz. of ferrous sulphate in 4.5 litres or 1 gal. of water plus a few drops of mild detergent). Spray as a very fine mist. If the treatment is successful the plants should begin to green up about 10 days after spraying. Foliar applications are a temporary measure and successive treatments may be required.
- Soil application of iron chelates: Apply iron chelates to the soil in the early spring by working them into the top 3 to 5 cm of soil around the base of the tree and watering well. Iron chelates are available from most garden centres. Soil application of iron chelates is the most effective treatment for chlorosis. Results, while not immediate, should last for one to two seasons.
- Soil amendment: Since iron is less available in soils with a high pH the addition of peat moss or the use of acidic fertilizers such as ammonium sulphate may help in mild cases. The presence of decomposing organic matter may also improve iron uptake. These methods are considerably less consistent than the use of iron chelates.
- Prevention: When the condition is known to persist in an area, then susceptible plants such as raspberries, currants, apples, high bush cranberries, roses and mountain ash should not be planted.
The browning of your rose’s branches can have many causes. Rose dieback is common, and can be the result of poor cultural practices, or of frost and severe winter damage. Dead branches should be pruned out and disposed of. When you take a close look at your browning branches, check for insect pests, grey mould, or black spots, cankers or mildew. This website gives a description of potential causes of rose dieback, and what to do: https://www.rhs.org.uk/advice/profile?PID=753