Asked August 24, 2016, 3:24 PM EDT
Thank you, Gillian Wright
Hennepin County Outside United States
Not sure I have enough information to say with any certainty. A six-foot shrub like this would require a very large container... 20" or wider. The conditions you describe... hot sun etc. are difficult for most plants, but even with regular watering can be especially challenging for plants of this size in a container. Watering would mean enough to see some begin to run out the bottom, but applied very slowly (especially if the surface has dried sufficiently to cause the water to run down the insides of the pot rather than wet the soil). One has to apply small amounts of water to ensure water is going INTO the soil, not around it, and then apply enough to make a difference. Of course I could have this backwards... I'm assuming droughty conditions and underwatering, but if you are watering every day... that is a lot if the pot is actually sufficiently large... so plant decline could also be too much water.
Are you intending to plant this forsythia in the ground? I don't know where you are writing from, but they require sufficient cold weather to induce blooming in the spring... which can be a problem in a pot causing the roots to freeze or go through excessive freeze/thaw cycles. A plant like this really needs to be in the ground. It should be planted 6 weeks (or more) before frosts in order to allow the roots to establish a bit
If you have images of the plant... close-ups of the leaves in trouble as well as general shots of its context, someone may be able to help narrow things down. If there is insect presence, images of the insects might also help. My first sense is that it is simply having difficulty being container grown.... or that that may at least be contributing to weakening the plant, making it more susceptible to whatever else is causing issues. Details about your location, growing conditions this year, whether an other plants are having issues near by.... also could help. Oh and what kind of forsythia is it?