Proving once again that there exists a video game for anyone's tastes, "Plant Tycoon" is a game released by Big Fish Games that puts you square in the soil-stained boots of a virtual gardener. The game allows you to plant, sell, and cross-pollinate various types of real and fictional plants in a virtual garden, and contains more than 500 species of plants in all. The "Venomous Grass" is a type of plant that can only be obtained by cross-pollinating ordinary plants with one another and is one of the more valuable plants available during the portion of the game when you first discover it.
Bring both your Fourpetal Astera and your Citrus Reptans to flower in-game. This can be done by keeping both plants well-watered and free of insects until they flower.
Select either plant, then click and drag the pollen from the "Pollen" box on the left side of your screen onto the other plant. This will cause a cross-pollination.
Wait until the plant you cross-pollinated has seeded, then click and drag the seed from the box in the left menu into a free soil-filled pot.
Water the seed. Once enough time has passed, it will grow into a Venomous Grass.