I have been growing Dahlias indoors (planted a few weeks ago) for planting outdoors in late May. They were planted in a standard potting soil and sit near a west facing window that gets about a half a day of sunlight. I have fertilized them twice since planting and I have taken them outside once for about 45 minutes on a warm day (15 degrees) and made sure they were not in direct sunlight.Some of their leaves are now turning brown and dry around the edges. It started on just one leaf and is now spreading to the others. Is this something to be worry about/something I can treat? Could I possibly be over watering them? Also, is it something that might spread to my other plants? I also have a number of seedlings started indoors that aren’t too far from the Dahlias and don’t want them to get infected if it’s a disease of some sort.
The good news is that that brown edges on leaves is not a disease but a symptom. It can be alarming but it’s correctable. The most frequent cause is too little water. The potting medium needs to be slightly damp (not wet) at all times. You may want to mist the plants to increase humidity. The second most common cause of leaf edge browning is over fertilizing. It is not necessary to fertilize until your dahlias are transplanted into the garden when a low nitrogen, high phosphorous fertilizer can be applied.
There is one other aspect of seedling care that should be addressed. I don’t know what Zone you are in but the Toronto area tends to range from Zones 5 to 6. The hardening off process should not begin until the soil temperature remains above 10 Celsius and the night time temperature does not drop below 15C. While we have recently had some warm days, the night s remain too cool to start the process.
Dahlias are stunning flowers and starting them indoors can be very rewarding but sometimes it takes practice to relax with their care and not hurry the process.