The first connection is how the plants get their energy. Deep in the interior of the star hydrogen fuses together to create helium also a small amount of mass is converted into energy. The amount of mass lost to energy is only 0.7% but the sun burns 600 billion tonnes of hydrogen a second. The energy released is in the form of light which can behave as a particle or a wave. The light bounces of particle in the interior for around a million years then after reaching the surface of the sun it only takes a 8 minutes to travel from the sun to the Earth. Then the plants absorb the light and with carbon dioxide from the atmosphere and water from the ground it creates starches and sugars that it needs, then some greedy animal comes along and eats all its hard work. The second way astronomy is connected to gardening is that ancient farmers used the stars as a form of calendar. If you observe the stars every night you notice that stars raise latter and latter every night and then earlier every night. You can use this knowledge to predict when it is getting closet to the time of planting, so you start to get your ground ready for planting. Another way astronomy and gardening are connected goes back to about 13.8 billion years ago. For the first few hundred generations of the plant the farmer keeps the seed of the best producers to get better and better crops. Before that in the wild the plants grow on their own and which ever is more fit to survive than the others get to survive to continue on their line. 4 billion years before that the atoms that made up the plant were swirling in a giant dust cloud that would be someday be the planets and the sun. Around 9 billion years before that there was a sudden flash of light that cooled down to create all the hydrogen that fuse together in the future generations of stars that would become the plant and the gardener. Now I wish I was a better at weeding.