The simple answer to this is......it depends :-) It depends primarily on the plant and its moisture requirements. A good many plants prefer dry soils - lavenders, rosemary, most ceanothus, many pines and junipers, lots of xeric species - and continually moist or damp soils can cause root rots or otherwise impact healthy growth. If the plant is properly sited for its species and growth requirements, roots will grow in any appropriate soil condition, dry or wet. Appropriate is the key word here....you need to match the plant to its preferred planting condition. Some want evenly moist soils year round, others like regular summer irrigation and dry winters, many others prefer the reverse and still yet, other plants, like those noted above, prefer to remain on the dryish side year round. And there are a few species that don't mind being in water - not just moist soils - on an ongoing basis. Establishment of new plantings is a bit different.....even very drought tolerant species may need supplemental watering for a few seasons to help them get established, but that can vary widely as well.

So it's all a matter of knowing your plant, the conditions they originate from and their specific cultural requirements and taking it from there and applying it to your particular soil situation(s).

Thank you Gardengal48. I thought dry dirt was too hard and impenetrable for roots to be able to grow and elongate and spread and thicken in it.

Thank you Gardengal48! So roots CAN grow in dry soil.

I don't have to feel so miserably guilty about not watering enough late last August and September. I was so exhausted doing nothing but watering and I was trying to let the plants "harden up." But when my yard guy dug deeper than an inch down, he said the dirt was rock hard and dry.

Should have installed irrigation first. Tried to, but every plumber who came out said since there was a fire hydrant by my water meter on my property, the codes were too strict and it was too complicated and they wouldn't do it. Neither would any irrigation company I called.

Should have kept searching until I found somebody willing to do it.

Fall is the time when hardy plants perform 40% of the annual total of root elongation. As mentioned, drought adapted plants will do this even when the soil is quite dry. It wouldn't be expected that a dried out, dying specimen of a monsoon climate plant would be able to mount a normal level of root production.

Although there could be exceptional plants, the way I'm reading your first post - the answer is no. Unless you mean soil that is starting to dry out. But in "dry" soil - no.

I read an article once that said recent research shows that roots do not grow "toward" moisture, but actually grow IN soil that is moist. asically, I think that might be splitting hairs as the two tend to occur at the same time. Roots - like trees - can grow deeper as the moist layer is lower and the top dries out.

If most plants could grow in dry soil, I think the term "permanent wilting point" could be erased from the books. It still depends on what plants - and if they are exceptional.

I'm not aware of native shrubs and tres on the west coast that I would expect to place in "dry" soil and expect any more than the root ball drying out, passing the permanent wilting point. Somehow I think you mean "on the dry side".

This reminds me of a photo I took of some adventitious roots in the upper needle mulch and humus around El Viejo del Norte last July - the 5th largest known coast redwood in the Grove of Titans.

There were two kinds of fine roots:

Dead ones.

Live ones.

The living, were from the previous autumn, winter and spring, presently just on the verge of dying. The dead ones, were from the previous summer's end, having already dried, died and shriveled.

Those were surface roots near the trunk. The sub-soil does not dry out around this tree.

Thank you Mdvaden. I have not been able to wrap my brain around roots doing OK or growing in dry soil. The summers here are so long and hot the soil definitely dries. If it does not rain much I cannot understand how those roots will do their autumn elongation march in bone dry hard soil. In August, September and October it's a challenge to know when to stop watering so plants can "harden off" and when to start watering again so there's moisture in the ground for the roots.

Lately it finally rained so much that the yard filled from underneath and the tree wells were little lakes. Then I knew the entire root balls had become wet. The drainage creeklets we dug are so effective that now within 24 hours after the yard becomes a standing lake, it has drained completely.

If only the excess winter rains would fall in spring and summer, enough to keep the soil moderately moist, that would be optimal. There are reports that the groundwater table is falling fast in Oregon and Washington and there will be water shortages. And one can see trees dying everywhere from drought stress. I'm surprised more ppl don't observe this and measure their rain gauge and take a scientific approach. Actually digging down into the soil is quite revelatory.

If the soil is completely bone dry, it's true - roots won't usually grow in that (with a few exceptions such as Opuntia spp.) But more often in the Northwest, soil that appears very dry usually still has a little bit of moisture left in it, and roots of established, drought tolerant trees and plants don't need more than that for good growth and development. But that level of moisture usually isn't enough for newly planted plants with root systems confined to a small rootball that quickly dries up.