Plants and Seasonal Sun: a Quick Gardener's Guide

Gardening labels are written for a generic sunny suburban backyard with no neighbouring houses. Yours probably is not that. "Full sun, six hours" sounds like a simple instruction. It is not, because the six hours of direct sunlight your garden gets on June 21st is nothing like what it gets on March 21st or December 21st. If you do not account for that, half your plants will sulk.

What "full sun" actually means

The horticultural definition of full sun is six or more hours of direct sunlight during the growing season, measured at the plant's location. The growing season part matters. In the northern hemisphere, that is roughly April through September. The seed packet is silently assuming the longest, highest-angled days of the year.

In April, the sun rises later, sits lower, and sets earlier than in June. The same spot in your garden might get five hours of direct sun on June 21st but only two and a half on April 1st. That is not a small difference for a plant that wants "full sun".

The four sun categories and their honest definitions

• Full sun: 6+ hours direct sunlight during peak season. Tomatoes, lavender, roses, most fruiting vegetables.
• Part sun: 4-6 hours direct, with afternoon sun preferred. Herbs, peppers, many flowers.
• Part shade: 4-6 hours direct, with morning sun preferred. Hostas, ferns in some climates, leafy greens.
• Full shade: Less than 4 hours direct, mostly dappled or indirect. Many woodland natives, mosses.

How to map your garden's actual sun pattern

Pick a sunny weekend day around the spring equinox (late March or so). Go out every hour from sunrise to sunset and photograph the same garden spot from the same angle. By evening you have a flipbook showing exactly when each part is in sun and when it is in shade. Do the same exercise around the summer solstice and again around the autumn equinox. Now you have three real data points, and you can tell which parts of the garden are "summer full sun, autumn part shade" versus "year-round full sun".

If you want a shortcut, use a tool like sunhour to look up the sun's azimuth at your location across the year. Combine that with a rough sketch of where your house, fence, and trees cast shadows, and you can predict the pattern without standing in the garden for fifteen hours.

The sun-angle effect

The other variable nobody mentions is intensity. Direct sunlight in late June, with the sun fifty or sixty degrees up, delivers far more energy per square metre than direct sunlight in late October when the sun barely reaches twenty degrees. Two hours of June sun and two hours of October sun are not equivalent. The plant cares about total energy, not clock time. The shoulder-season "sunny" days are weaker than they look.

Wikipedia has a good page on solar irradiance if you want the maths. The practical version: do not be surprised if a south-facing spot that thrived in summer suddenly looks anaemic in September.

Latitude matters too

If you garden in Singapore, the seasonal shift is small. The sun is always high, the day length is always close to twelve hours. "Full sun" is consistently full sun. If you garden in Berlin, the same spot is full sun in July and full shade in November. The further from the equator you are, the more aggressively you need to time-share plants across the seasons.

The practical move

Plant year-round perennials in the parts that are full sun in spring and autumn, not just summer. Plant high-demand summer annuals in the parts that are full sun in summer only. Plant shade lovers under the deciduous trees that will leaf out in May. The garden has microclimates inside itself; the trick is to match plants to the climate they will actually experience, not the one a seed packet imagines.

The general rule

If a plant labelled "full sun" struggles in your supposedly sunny spot, the problem is almost never the plant. It is the silent assumption that your sun is the same as June sun. Map it once, properly, and you stop guessing.