Our harlequin maple is 25 years old and has always been beautiful with green and yellow outline leaves. We know know to remove entire green leaves so the whole tree doesn’t become green leaves. This season (we live in Ottawa) there is very little yellow edging on the leaves. Is it because I should have been giving the tree water – with a lot of rain this summer I did not give it extra water. As well I gave it potash last fall as it was recommend by a local nursery to do so in Oct and I did not do for a number of years because I could not get it locally and then found it. Why are the leaves not as prominent this year and should I give it potash in mid Oct. Your advice is greatly appreciated. I am not a resident of Toronto but can you still help me. I just saw that below.
Thank you for contacting the Toronto Master Gardeners.
Harlequin Norway Maples, Acer Platanoides ‘Harlequin’, make wonderful additions to a garden where there is room for them to grow to maturity (100 years and 40-50 feet), or to be kept pruned for a smaller urban garden specimen.
With regards to care of these trees, below is some general information:
- Hardy to zone 4a so should be able to withstand an average Canadian winter.
- Full sun is the preferred planting location although it will tolerate some shade.
- These trees like well draining soil but are fairly adaptable.
- Pollution tolerant, so they are good with an urban environment.
- Maintaining a medium moist soil level is important for optimal plant growth.
- Pruning out of all/most of green non-variegated leaves in the spring so tree doesn’t revert to it’s all green form.
To address your particular tree, two things jump out of your enquiry as potential issues.
Firstly, this seasons reliance on mainly rainfall to water the tree. Even though it has been a wetter summer in parts of the province, these short bursts of heavy rain are not nearly enough to penetrate the soil surface to reach tree roots. This situation is basically long periods of drought for deeper roots and only moisture for roots closer to the surface – too little water for a 25 year old tree. Watering deeply a couple of times a week during the growing season is highly recommended – a drip hose/irrigation is ideal for this job.
Secondly, your addition of only potassium K (potash) fertilizer. Supplemental Potassium is generally used to promote strong root growth, improve soil moisture uptake and promote overall plant health, so is used mostly for flowering and fruiting trees (orchards etc..). Since your tree doesn’t fall into this category and is more mature, you may be inadvertently causing issues that you haven’t had before. Below are a few examples of what can happen if there is an excess of soil Potassium:
- Can increase soil pH making it more alkaline, which could hinder the trees ability to take up nutrients.
- Excess will affect mature tissue first as the mineral gets accumulated here.
- Potassium toxicity can cause a deficiency of Nitrogen N and Calcium Ca being available to the tree roots as well as having diminished levels of Manganese, Iron, Zinc and Magnesium. These macronutrients and micronutrients are vital for plants to thrive.
- An application of Potassium to the soil can take several weeks to several months to break down so that it can be available to the roots. An October application would not have any impact until spring when the tree is flushing out with new leaves.
So here’s our advice:
- Water the tree deeply now and regularly up until freeze-up as this will give the roots moisture to take it into and through the winter. It will also help flush out any residual potassium from the soil, correcting the imbalance.
- Do not feed it with any more single potash fertilizer (not now, not in October, not in the spring), as your tree doesn’t need it. If you wish to feed the tree in the spring, use an all purpose fertilizer 20-20-20 (N, P, K), so that you are providing a full spectrum of macro and micro nutrients.
- In spring, make sure you prune out all full green leaves and if you can’t reach them all due to the height of the tree, consider hiring a professional.
Hopefully this information is helpful.