Hello Maggie,

Thank you for reaching out to Ask Extension with your questions about Fine Line Buckthorns. 

 My experience with Fine Line Buckthorns is that they are slow growers to begin with, so it might take some time for them to reach full maturity height. Would you be able to send a picture of your Buckthorns? I'm curious where they are planted and if they are mulched with wood chips or rock...

For most plants, (especially perennials, but it can be true for woody plants also) we say that the first year planted they "sleep", the second year they "creep" and the third year they "leap". A lot of this has to to with becoming established. It can typically take newly planted perennials, shrubs and trees in our region a few years to grow enough new root mass to be considered "fully established". 

If this is the third year for your Fine Line Buckthorns and you have noticed that they still are not growing in size from when they were planted, it could be from several factors. They could have been planted too deep, they could have other root issues (soil compaction, girdling roots, etc), they might not be getting enough water, they could be struggling in the high heat we have had this season (My #1 guess), they could have been damaged in the record low -20 temperatures we've seen the past two winters. It is hard to tell, especially without seeing what they look like. Are there any signs of stress on these plants?

Sometimes you can encourage plants to grow with pruning. This idea can be confusing to people who want their plants to grow bigger, not become smaller. But pruning typically promotes growth, so if you snip off just the last leaf at the tip of a few branches, then those branches will grow much quicker with new growth than if you just left them alone. You might try this in a few spots on one of them to see how the plant responds to the pruning.  Tie a string or use some other marker to keep track of which branches you snipped, so that you can watch how the plant responds with new growth. This can make shrubs grow more full also, and I'm betting it's more height you want with these columnar plants rather than them becoming bushier and fuller, so try only a few spots and see if the new growth is satisfactory for what you want these shrubs to do. 

Have you ever fertilized them? If not, then it would be a good idea and could also help. Right now is too hot and not the right time to fertilize plants in our region. But in the fall after leaf drop or next spring before they leaf out again,  you might think about giving them some fertilizer. Here is more information about fertilizing:

https://planttalk.colostate.edu/topics/trees-shrubs-vines/1720-when-to-fertilize/

https://planttalk.colostate.edu/topics/trees-shrubs-vines/1708-fertilizing-trees-shrubs/

I hope this helps,

Dawn