Dahlia Help!

RosieG

Hi, I potted some Dahlia tubers the first week in March and left them on a sunny windowsill. They were coming on great and I cut them down a couple of weeks ago and put them outside in my walk in greenhouse. Some of them look like they are willing and the leaves are turning brown and some of the main stalks look hollow. I'm not sure what I'm doing wrong but I would love to be able to save them! I'm not sure if its the weather as it has been cold at night, I've been leaving them out in the sun through the day then last couple of days but nothing seems to be helping. I fed them with tomato feed yesterday. Any help would be appreciated! Thank you!

Daniel Rutherford

I’d say it’s cause of the cold, they hate it too cold especially if any frosts hit them. Mine are still inside at the moment and won’t even go in greenhouse until weekend at least.

They are just very tender plants and need to be protected while young in spring, don’t put outside at all while cold persists would be my advice but hard to tell if it’s too late now

SophieK

Not a dahlia expert, rather a beginner, but perhaps they have been getting too much real sun? The leaves of house grown plants are rather thin and actual sun can scorch them easily. When you harden off a plant, you need to give it an hour of full sun for a couple of days, then extend little by little the duration over a period of 10-14 days.
As far as the main stalks being hollow, it's quite normal with dahlias.

Posy

You are doing far too much to the poor things, @RosieG. The best way to start them off is to pot them up and keep them in a bright, cool, frost free place. A little direct sun will not hurt, but avoid big temperature changes. Do not cut them back, feed them or move them around, it's much too early. If your greenhouse is heated, they will be fine there but they hate cold so otherwise, keep them indoors. When we have no more frost they can go out in the day and in at night for a week or so, then plant out. I expect yours will recover - good luck.

Butterfly66

You dont need a heated greenhouse. Mine start off in my unheated one, not as fast as they might be with extra heat but absolutely fine. 

As we have a peach tree in there as well the door is left wide open on sunny days January onwards to help with disease control and get the pollinators in, Dahlias manage fine

Nanny Beach

Posy has nailed it, if you are in the UK, it's about 6 weeks too early to put them outside, they are very frost tender, and if they get wet at all, even worse.Normally packets come with warnings not to plant out until after frosts.  Normally I do the Sarah Raven method, which is pot them up in March, exclude all light for at least 10 days, I put plant saucers on mine, then have a peek, if you have growth it will be very pale, then leave the top off, if no growth cover back up.  Hubby did ours this year, potted and in a greenhouse, and guess what one one has shoots! I think folk are often confused by what counts as a "heated greenhouse", no heat is only frost free, we have a tubular heater in one, thats a "cold greenhouse", not a heated one

Posy

If the temperature in your greenhouse falls below freezing,  young dahlias will suffer. In some areas, with no heat at all, you should be safe, but if the temperature outside is  say  -5 you will be lucky to escape damage inside. Greenhouse terms describe unheated, frost free and heated and I was lazy saying heated when I really meant frost free. Young dahlias need to be frost free.

Butterfly66

You’re probably right @posy. I was also lazy and should have caveated that it may vary depending on your local climate. It has reminded me that it would be interesting to have a thermometer in our greenhouse so we can see just how cold it does get. Although I haven’t anywhere else to put anything if I did know it was “too cold” so maybe best not knowing 🤔

Interestingly , I took a gamble and simply left the dahlias in their summer pots to overwinter in the greenhouse. They went in there initially to dry out ready for decanting and storing. A bout of flu put paid to any progress by which time we had had several hard frosts so left them assuming the damage would already have been done. Thought I would have lost them but they have all come through largely unscathed, a few soft tubers at the surface which I’ve pulled off, and are just starting to send up shoots. Am still undecided as to whether to take the same gamble next year...

RosieG

Thanks for all your help, I will move them indoors and I really hope they will recover!! Also I keep reading conflicting information about how much to water them, do you not water them until they sprout then after that just once or twice a week? Many thanks!