The vernal equinox is special for gardeners in the Northern Hemisphere because it signals the beginning of spring, just as the autumnal equinox signals the start of spring in the Southern Hemisphere. Because of the earth's tilt, our days will get longer and the sun will rise higher in the sky. That change in sun position affects your garden and all of the plants in it.
If you haven't before, begin looking at the shadows in your landscape. Make note of shadow creep in coming days. The shadows that caused snow to turn into ice and stay frozen will shift to new zones. Shadows will creep into spots they haven't covered in months. Areas that basked in the winter sun may soon be awash in shadows with the shifting sun and growth of tree leaves.
Without this foresight, tomatoes, peppers, cucumbers, squash, and all the other sun-loving plants may have difficulty producing efficiently. Just a few hours of shade during the day can affect fruit and vegetable production in a plant that needs a full day of sun. Think about your gardens in years past. Was low harvest a result of too much shade?
Unless you spend a year marking shadow lines, it may be difficult to select the perfect location of a new bed. You can estimate shadow creep by watching closely over the next few weeks. Place a tall stake in the ground and observe the shadow throughout the day. Do the same thing tomorrow and again next week. Every day you should notice a change. The sun will gradually move to the north and the shadow will move to the south. There will also be a shift to the east and west depending on the time of day as the sun is higher in the sky. If you don't use a stake, look at a tree. Observe the shadow's location on the ground.
Because you'll be planting soon, you need to guess where the sun be be in three months and how shadows will move across your garden. If you have a deciduous tree on the north side of your garden, you may think little of it because the sun has been shining from the south. But as the tree fills with leaves and as the sun creeps north, it may shade a portion of your garden in the morning or maybe in the early evening. That's not a problem if you have plants that like a little early or late shade, but it is for a plant that needs full sun.
Ideally, use the next three months to accurately mark the movement of shadows in your garden. The next three months are important because shadows will change each day until we reach the Summer Solstice, at which point the sun reaches its highest point and shadows will start retreating back to where they are today.
Think about your seeds and plants before you put them in the soil. Take a moment to look around at trees and sheds and fences. Look at your shadow and look at the shadow of the tall structures around your garden. Imagine the creep of the shadows over the course of the next three months. If your new plant won't get all the sun it will need, consider putting it in a different location.
Most gardeners focus on the sun when they plant. With the sun comes shadows. Without considering both components you're leaving out critical information so observe the entire process. Today is a good day to begin.