Nature is the Teacher

I've been thinking about why it's so cold so late in the spring, and what the effect is on the plants. The explanation for the cold is easy, a region of high-pressure near New England has set up a clockwise flow of air that features winds from the Atlantic ocean. Since the ocean is cold this time of year, still in the 40s, the steady breeze off the water keeps Boston cool, even cold this time of year.

The cold and its effects on the plants is a little more complicated. First, plants that are close to the ground probably experience quite warm conditions this time of year. The soil is steadily heated from the sun and at night, because temperatures are relatively mild (influenced by the steady ocean temperature), there is not drastic cooling. So plants near the ground, which experience little wind, experience warm conditions. This may explain the abundant growth of groundcover plants and the exuberant bulb activity this time of year.

The higher plants grow off the ground the more cool breezes they experience. In general, we see a lot of flowers and the flowers, which don't have highly developed vascular systems (at least not as highly developed as the leaves...flowers are temporary after all), benefit from the cool temperatures. So this year we had a mighty showing of our star magnolia that lasted longer than usual. It also helped that the delicate flowers weren't caught in a downpour, which seems to happen most years. 

The young leaves of trees and shrubs benefit from cool weather because they have not hardened off yet. Their vascular systems are still developing. Because they are not fully open yet other systems, such as cells that block loss of water, are also not fully developed. And most leaves have not yet formed a well-developed cuticle layer, which would also protect them from desiccation. So while we don't necessarily enjoy the "air conditioning" off the ocean the young leaves and plants generally benefit from it.

This year has been a bit unusual in that we haven't had any significant moisture for a few weeks. We expect some in a day or so and it should last for several days. When this happens there is increased danger for the plants of invasion by fungi and other organisms that damage the young leaves. One example of this is anthracnose of sycamore, a disease that strips the first flush of leaves from the trees. Anthracnose is severe here in Boston, where sycamores (Platanus) are near the northern edge of their range. 

This is the time of year when other pathogens may enter the leaf, either through the open stomata that provide gas exchange for the leaves, or through the partially developed epidermis that has not yet "hardened." Some of these pathogens will remain dormant or slow-growing within the leaves and will not become apparent until late in the season when the leaves have begun to senesce in the late part of their annual life cycle. 

It's just amazing to have a chance to observe the responses of plants to the seasons. Nature teaches us so much about itself through simple observation. This light-infused time of May is to me, the most inspiring of all. If only it would warm up!