About "foliar" fertilization of evergreens
We have dwarf blue pine and various junipers growing in our courtyard. We also have deciduous bushes growing there. The courtyard is underlain with sheets of black plastic material to prevent (or at least minimize) water infiltration into our sub-grade parking facility under the courtyard. The plastic has been installed right up to the plants, so that the drip zone is largely impenetrable to surface applications of fertilizer. Fertilizer spikes would be frowned upon because they would damage the plastic liner. I have decided that the foliar application of liquid fertilizer is the best solution for the deciduous plants. I have no information how well "foliar" application might work on evergreens. Do the needles absorb fertilizer much as leaves do? Is it worth a try? A suggestion has been made to me to flow a dilute liquid fertilizer solution down into the root zone near the trunks of the plants (where there is an opening in the plastic liner) and allow the fertilizer to diffuse into the feeder roots. There seems to be enough moisture under the plastic cover to keep the plants alive. (Also, water leaks into our sub-grade parking space when the snow melts or it rains.) I cannot imagine the damage to shrubs and evergreens if fertilizer pools up in areas near the trunks and am hesitant to begin this mode of fertilization. I am hoping foliar spray is the answer. Please advise. Also, what do you call it when you apply fertilizer to needles. My spell-checker does not like "needular."
Hennepin County Minnesota
Foliar applications of fertilizer are not a good way of providing nutrients in the long term. We recommend them for quickly dealing with nutrient deficiencies and pH adjustments. Read here about fertilizing trees:
Actually, I think fertilizer is the least of your problems. Black plastic prevents water from reaching the root zone. It chokes oxygen away from the roots and any microorganisms growing in the soil. Fortunately for the plants, black plastic eventually degrades and develops holes.
I'm not a construction expert, but since black plastic degrades and is an unreliable water barrier, I don't think it is a good way to prevent leakage into an underground garage. If you have leakage into the garage, then there are probably structural problems that no amount of black plastic will fix.
It seems you have competing wishes. You want an impermeable water barrier for your garage, but not for your plants.