I live in Thornhill. I very successfully over-wintered several pots of geraniums. Once this week turned warm and sunny I placed them outside, on the east facing steps of my house, where they had grown all last summer. After several hours they appeared as if a blight of some sort had hit them. Very scary! I pruned the damaged leaves and am hoping they will revitalize. This didn’t happen last year, and I also had success last year, drying and storing geraniums in the basement then replanting them in the garden. They thrived. About to replant the ones that weren’t in containers. Thank you for your consideration!
Thank you for contacting the Toronto Master Gardeners about your geraniums. Please read the advice given on a previous published response.
Moving plants from a relatively stable indoor environment to the outdoors where the sun is strong, temperatures vary and the wind can buffet the plants, is stressful.
Your geraniums are suffering from sun burn. This is why indoor plants should be introduced gradually to the outdoors using a process called hardening off.
This involves putting plants in a shady spot sheltered from the wind for the first few days of being outside, and returning them inside for the night. Then, gradually introduce the plants to full sun, and full time outdoors. Make sure that your plants are well watered. Avoid any fertilizer for the first week or two outside.
I recommend you start following the hardening process describe above.
Growth of your plants will be delayed for a few weeks, but they will hopefully recover if there are enough unharmed leaves or leaf area to continue to photosynthesize and produce food.
It is better to leave the scorched leaves on the plant unless they are no longer green. Any dead leaf material should be removed to avoid becoming a magnet for plant diseases.
Additional considerations before setting your indoor plants outside permanently is the last-frost day and the night time temperatures. Generally the last-frost date for Toronto is May 11-20; and the geraniums only flowers when the temperature is at least between 10-16 C degrees. Plants that have spent the winter indoors do not like night time temperatures below 10 degrees Celsius. You might consider setting the plants out during the day for the next week or so, and bringing them in at the end of the day. Set the plants outside permanently when the weather is forecast for warmer nights.
As noted in another published response, a further consideration is acclimatizing your plants to the outdoor conditions. Plants coming from a Nursery or Garden Centre will have been ‘hardened off’ already. But if they spend days or weeks indoors before planting outside, a gradual re-introduction to the new environment will help to prevent stress and improve your overall plant health. The hardening off process is explained in this Toronto Master Gardener Tip hardening off plants
Best of luck.