I usually recommend adding a quality flower food in water in which Oasis florist foam is soaked. Skip this, however, when you are using evergreens. Florist food can sometimes turn greenery yellow.
Do, however, process your fresh flowers correctly, using florist food in the buckets in which the flowers rest and rehydrate. Do not, however, put Crystal Clear in the greenery containers.
Allow the foam to "float soak" and take up the water naturally. Do not force it underwater trying to hurry the absorption or you may cause dry pockets that can be death to a flower stem resting in it.
I always use Oasis florist foam. I've seen a online blog where a DIY crafter said she compared cheaper florist foams against Oasis and the cheaper ones did better. I don't know where she bought her foam or how old the bricks were - but I've been a florist since the mid 1990's and am not persuaded that cheaper is always better.
I managed a large shop in my past and we used a lot of different brands that different salesmen would give to us. It would take more that a one time comparison to convince me that cheaper foams are better than Oasis.
Selling Oasis is my choice. I'm not paid to be the spokesman by any floral supply manufacturer. I recommend the products that I use myself and know how they perform.
My other concern is the availability of overstock florist supplies through found at cheap discount stores, dollar stores or on an online auction sites. I suspect this stock is sometimes purchased as buyouts from warehouses that have closed, etc. As foam gets older can begin to degrade and not take up water as it should.
I recomend that if you are buying for a large event (like a wedding) that you purchase from a reputable and active professional floral distributor whose stock turns over on a regular basis.
You'll need one brick of florist foam cut into approximately a third to fit this container. The Oasis is scored lightly to make easy measurements.
Float soak this piece along with the Oasis cone.
Make the piece large enough to completely fill the bottom ceramic container that will serve as the base for the tree design.