Huge, majestic trees are just one of the reasons that many of us chose to live in Del Mar. Our beloved Torrey Pines are unique to this part of the southern California coast and Santa Rosa Island, just off the coast. Our many Eucalyptus varieties were planted by Santa Fe Railroad magnates, who thought the wood from them might be useful for making railroad ties. Since the wood from them twisted too much as it dried, they weren't appropriate for ties, but they have remained as prominent landscape features nonetheless. These two favorites as well as large specimens of other varieties (have you noticed the huge Moreton Bay Fig at the top of Ninth Street?) shelter Del Mar with a green roof, beautifully framing our ocean views as they reach for the sky.
Many people complain to me about the inability to grow other plants under these mature trees. This can be a mighty challenge for several reasons.
A mature tree that has thrived for decades has developed an extensive root system. Wide-reaching roots are very efficient at taking up available water and nutrients. For this reason, an existing tree has a competitive advantage over new plants,.
Next, existing trees may cast deep shade, cutting off much needed light from other plants. While understory plants may have once grown in full sun or partial shade, their environment becomes progressively shadier as the trees above them grow and they no longer receive the light they require for photosynthesis.
Furthermore, a constant rain of leaf, needle and bark litter can smother understory plants making it impossible for them to survive.
Finally, in the case of Eucalyptus, resinous toxins in their leaves may make it difficult for seeds to germinate in the soil around it. Recent observation, however, shows that competition is the main factor limiting successful underplanting.
So, how can we deal with this problem without sacrificing our beautiful trees?
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First, choose varieties that have a relatively low need for water and nutrients. Many California natives, such as Rhus integrifolia (Lemonadeberry), Ceanothus (California Lilac), Agave and Arctostaphylos will thrive under Torrey Pines and Eucalyptus. Add to this list some of the varieties we have imported from other Mediterranean climates. Cistus (Rockrose), Westringia (False Rosemary) and Leptospermum (Tea Tree), Crassula (Jade Plant) are all very drought tolerant and not dependant on a heavy dose of nutrients.
Second, chose varieties that can grow in acid soil. Because of the abundance of leaf litter, the soil under large trees is usually relatively acid because of the decomposing leaves and needles. Many plants in the Rhododendron family prefer acid soil and will grow quite well under pine trees, providing that they are given enough water.
Third, plants with large, coarse leaves and an open structure will allow tree leaves and needles to drop through them to the ground rather than accumulating on top of them. For this reason, open shrubs are a much better choice than a tightly knit ground cover.
Finally, the soil under mature trees is filled with roots and it's frequently almost impossible to dig a planting hole under them. Think about leaving the leaf and needle litter from the trees to cover the bare ground and use large pots (greater than 18” tall and wide) filled with shade-loving plants instead of planting in the ground. These potted plants will have their own source of nutrients from the potting soil. A drip system can be rigged up with emitters supplying water to each pot. Camellias, Azaleas, Anemones, Fuchsias, Gardenias, Ferns and made other shade-loving plants are all great choices when competition from large tree roots is eliminated. It's even possible to combine several plants in each pot to assure blooms in all seasons.
It took many, many years for our beautiful trees to grow to the size they are today. They deserve respect for their efforts and the beauty they bring to our environment!