The raiders broke in and drove a lorry into the rear compound before using the hydraulic hoist on the vehicle to load up the trees.
The incident happened between 6.50pm and 7.05pm on Wednesday 25 November.
Stewarts representative Susie Stewart said: "We are bitterly disappointed and disgusted by what has happened. We all work really hard through the year on our Christmas stock and we put a lot of effort in. It's disappointing — mainly for the staff who work so hard."
Police are appealing for witnesses and anyone with information to come forward.
Anyone with information is advised to call Dorset Police in confidence on 01202 or 01305 22 22 22, or the free and anonymous Crimestoppers line on 0800 555 111.
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