Design: Plant Growth [3, 6 5]
We have some experience bringing life into the world - primarily through wasps, but it's experience nonetheless. As it turns out, making things grow is pretty dang easy compared to spewing blood-thirsty wasps out of your fingertips.
The spell is cast through a wand of fresh-cut dogwood, still green and sticky with sap. It requires rather precise cutting, engraving, and enchantment before it can be used, but dogwood trees are common and a mage could produce a wand a day, given the time and motivation. The downside is that the wands only last a couple days before they dry out and stop working, but that's not a big issue due to the relative ease of production.
The effect is currently constrained to what our mathemagicians refer to as "low-level" plants - grasses, weeds, wildflowers, plants of that nature. Each plant must be enchanted one at a time, with each enchantment taking a couple minutes to perform, but once enchanted the plant can grow from a sproutling to full adulthood over the course of single day. Aside from the eventual combat applications, our researchers are excited as to how this spell can be used in civilian life. Poor harvests may one day be a thing of the past, and we will never want for wood to keep our homes warm at night. Imagine growing tropical fruits in the dry, cold tundra of our homelands. Oranges and pears for breakfast and dinner every day, rose gardens lining every road in Arstotzka, full granaries of wheat and corn every autumn - our King seems particularly pleased with the direction of our research. It still needs a bit of work, but our current progress is very promising. Cheap.