Where the Rain Doesn’t Reach

August 10th, 2021

   Those summer rains can be a little tricky when it comes to keeping plants happy.

Just because we get a heavy rain doesn’t mean it’s reached all of our plant roots.

   Just because we’ve had a downpour doesn’t mean water has made it to where it counts, which is down to just below and all around our plant roots.

   Young and newly planted plants are particularly susceptible to these fakeouts.

   One problem spot is where the rains don’t reach at all.

   Tops on that list is under roof overhangs. These can sometimes reach out two or more feet over walls, blocking rain from falling on anything you’ve planted that close to the structure.

   Keep an eye on plants in these spots because you might have to water them even in rainy weather.

   A second sneakily dry area is under trees… especially big shade trees.

   For one thing, raindrops can be blocked and/or absorbed by the thick leaf canopies, limiting how much water actually makes it to the ground.

   Then the rain that makes it through the canopy has to penetrate the mulch layer that most people have around their plants under trees.

   Depending on how thick that layer is (and it shouldn’t be any more than about three inches around shrubs and one to two inches around perennials and groundcovers), it can take an inch or more of rain just to reach the soil surface under the mulch.

   Rain that gets soaked up by mulch is a factor anywhere, by the way… not just under trees.

   Even when enough rain falls to adequately dampen the soil in the root zone, big tree roots can quickly grab it, easily out-competing small and younger plant roots.

   Check your soil under trees sometime after a rain and see how much of the water has actually made it to paydirt. And keep the hose handy whenever planting new plants under trees to help compensate for the rain-blocking and root-competition.

   A third issue is on banks and slopes.

   Summer storms often happen in short-lived deluges rather than gentle day-long soakings, leading to water that can run off rather than soak in.

Dry, knitted mulch can repel water.

   If your bank is covered with shredded hardwood (the most common type that also offers the benefit of knitting together to stay in place on a slope), you might find that rain runs off right off the bat. That’s because this knitted layer makes a tight surface that repels water until enough of it falls to break the surface tension and soak in.

   South- and west-facing banks dry especially fast, so even when a good rain manages to soak the soil into the root zone, these can dry back out within a few days.

   Plants in pots can dry out even faster.

   Even after a good soaking rain, the soil in your containers could be dry again a day or two later.

   As with all plant-checking, your index finger is as good as any gauge in assessing how damp the soil is a few inches down.

   With pots, tipping them a little to assess their weight is another option. Dry pots are noticeably lighter than damp ones.

   Those summer storms are way better than nothing at all. But keep in mind that they don’t necessarily get you and all of your plants off the hook for very long.