In normal mode I use greenhouses to fuel steam engines. While costly this ensures a steady power supply without eating valuable and rare coal.

I tried the same in marathon mode but there this looses energy. With the inserters and the assembler for seedlings producing wood costs more than burning it gives back.

Is this intentional? It kind of makes greenhouses useless, if not outright harmful, till you have researched boilers and steam engines mk2.

There is more use for wood than to simply burn it for power...

But try pure-speed beacons. I've got a mk2 beacon with speed 4s boosting fields of 24 greenhouses, and even the most basic arrangement gives, IIRC, 240% wood production for 20% more power input.

Note that some said recipes were not there at the time of the author creating the post.

Considering only red and green packs and Marathon, yes. The solution is using solid fuel from coal and hydrogen instead (%130 energy compared with just coal) or using other mods like Bio Industries, charcoal, AAI Industry, Black Market or wait until. Here are the calculations:

10 raw wood = 10 seedlings + 20 water (90 sec) in greenhouse (crafting speed of 0.75, power of 100 kW)
2 seedlings = 1 raw wood (0.5 sec) in blue assembler (crafting speed of 0.75, power of 150 kW) (In practice, it is random between 1-3 raw wood but in average it would give you more than 2, 2.055 in my test).
So, 10 seedlings need 5 craftings with 0.75 speed of 0.5 sec 50.5 gives sec /0.75 divided by machine CS gives 3.33 actual seconds * .15 MW = 0.5 MJ.
So, spent 0.5 MJ in seedlings, water is free; greenhouse is working 90 sec with CS of 0.75 means 900.100/0.75 = 12 MJ. So we produced 5 raw wood in net (5 went in seedlings) having 20 MJ. Having spent 12.5 MJ means it is only beneficiary if we can produce more than 7.5 out of the 20 MJ, or having an efficiency of %62.5 or higher (disregarding all logistics and inserters).

However, at the same level of technology, regarding preciousness of coal in the first place, you could use coal to have solid fuel.
1 Solid fuel = 1 coal + 175 Hydrogen (3 sec) in chemical plant (crafting speed of 1.25, power of 210 kW). Coal is 0.525 sec with 90 kW (0.1714 MJ each).
10 pure water = 20 Hydrogen + 12.5 Oxygen (will be discarded) in 1 sec (crafting speed of 0.8, power of 460 kW). 175/20 gives 8.75 craftings, / 0.8 gives 10.9375 actual seconds * 0.46 gives 5.03 MJ.
Chemical plants works 3/1.25 sec (2.4 sec) with 210 kW needs 0.504 MJ.
In total, 0.504 MJ + 0.1714 MJ + 5.03 MJ = 5.71 MJ giving 25 MJ compared with 0.1714 MJ giving 8 MJ. 19.29 MJ with solid fuel compared with 7.83 MJ with coal.