1 - Container grown trees may be planted in the summer and will get some help from the monsoon rains, but don't trust the monsoon for adequate irrigation.

Yard and Garden June 17, 2017

Q.

Can I plant pine trees during the summer? I want to replace some trees that died several years ago. I want to plant them at a camp in the Manzano Mountains.

- Scott T.

A.

Container grown trees can be planted most of the year. New Mexico monsoons can help trees establish, but you cannot count on the monsoons to provide adequate water, even in a good monsoon year.

The first thing to consider is the size of the trees to plant. Smaller trees will establish more quickly than larger trees. The smaller trees will have less water loss through a larger canopy. This will reduce the stress on the tree and allow a more rapid development of the root system and subsequently a more rapid development of the tree. In a few years, the smaller tree may overtake a larger tree for this reason.

Select trees that are well rooted in their pots, but which have not developed an extensive circling root system in the pot. If roots are circling inside the pot, it will be necessary to cut the circling roots to reduce the chances of roots girdling the trunk as the trunk enlarges. If some roots must be cut, make clean cuts that will more quickly close the wounds and develop branch roots behind the point of the cut.

To assist the tree in establishing its root system, prepare a large “planting site” that is much larger than the pot the tree is currently growing in. For even a small tree, a 3 foot or greater diameter prepared planting site is good.

To prepare the planting site, first loosen the soil over the whole planting area to a depth no greater than the depth of the root ball in the container containing the tree. This leaves naturally compacted soil under the tree and prevents the tree from settling as it is watered after planting. Planting a tree too deeply or allowing it to settle deeper after planting will slow development of the root system and development of the top of the tree.

If the soil at the planting site is very sandy or high in clay content, addition of decomposed organic matter (compost is better than manure) will help the soil allow entry of water, increase water holding capacity, provide slowly released nutrients, and provide for more rapid root system development and tree establishment. You may use compost in other soil types as well, but do not add too much. Your goal in establishing the tree is encourage root expansion from the planting site into the surrounding native soil. If there is too much compost in the soil at the planting site, the roots may begin circling as they reach the naturally compacted, unamended native soil beyond the prepared planting site. Amend the soil throughout the planting site with compost, not just in the backfill soil around the root ball of the new tree.

Do not just wait until the planting hole is filled with soil to water. As you fill in around the root ball with backfill soil, water several times to help settle the backfill soil and fill air pockets.

After the tree is planted install an irrigation system if you will not be manually watering until the tree becomes established in two to three years. However, when installing an irrigation system, remember that initially the root system is small and confined to just the site of the root ball from the container. However, this root system should quickly begin extending outward and the irrigation system must provide moisture for this root extension. If the trees are in a location that will provide enough natural moisture in most years once the tree is established, the irrigation system need not be permanent. However, because New Mexico can experience extremely dry conditions some years, maintain some means of watering in bad years even after the tree has established its root system. In the first two growing seasons, the trees should be irrigated once a week and once established once every two weeks during the growing season. The newly planted tree and established trees should do well with once monthly irrigation during the dormant season. In the Manzano Mountains that you described, winter moisture may be adequate even the first year and after the second growing season very little irrigation may be needed.

Finally, after installing the irrigation system, apply a thick layer of organic mulch. This may be pine needles, wood chips, or bark chips. Keep the first few inches from the trunk clear of mulch, but otherwise apply a four inch or greater thickness of mulch over at least the extent of the prepared planting site soil. This will reduce evaporation, minimize weed growth, stabilize soil temperature, and facilitate permeation of irrigation and natural precipitation into the soil. Be sure that natural precipitation is sufficient to penetrate the organic mulch layer and moisten the soil to a depth of at least 2 feet with each irrigation or precipitation event.

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.

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