Improving resilience of planted native woody seedlings to drought — Fact sheet

This research by Manaaki Whenua – Landcare Research investigates different ways to help seedlings survive drought conditions.

Why is drought a problem?

New tree seedlings need water to survive. A lack of water, as happens in droughts, can cause water stress in seedlings.

To avoid water stress, new tree seedlings are planted from autumn to spring. During this time the soil is moist. This allows bigger root systems to develop before the dry summer months.

Tree seedlings planted outside the autumn to spring timeframe are at risk of drier conditions.

What causes planting delays?

The COVID-19 pandemic caused significant planting delays in the forestry sector. Restoration and revegetation projects were also affected.

Climate change will cause delays. As the weather changes, the ideal autumn to spring planting time gets shorter. 'Shoulder season' planting will become more common. This is planting that takes place on either side of the ideal planting timeframe.

Climate change will increase the time between rainfalls. This increases stress on new seedlings with small root systems.

What we did

We tested ways to lessen the effect of drought on trees planted at ‘tough’ sites.

Tough sites are places where:

  • planting has failed before
  • there is water stress
  • shoulder season planting happens.

By testing on tough sites, we found a variety of ways to reduce water stress on planted seedlings.

5 steps to planting success

All steps rely on the success of each other and must be done well. If one step fails, there’s a risk of poor establishment which puts the seedlings at risk.

1. Assess the site

Assess the site first. Looking at the soil shows you important factors for:

  • root growth
  • water behaviour
  • depth of mulch (decaying material spread around or over a plant to enrich or insulate the soil)
  • depth of thatch (the layer of dead and living plant material that forms between the soil surface and green vegetation).

When looking at the site think about the following.

  • Will the direction the land faces affect water stress?
  • Is there existing shelter (nurse trees)?
  • Are there sections of the site that are better for planting than others?

2. Assess the plants

When choosing plants, think about getting species tolerant to drought.

Learn what to look for with nursery stock.

  • Are root to shoot ratios high enough? This varies according to what you’re planting. Plants with bigger root mass and smaller tops are more drought resistant.
  • Is there good root development? Ideally with roots throughout the pot without being
    in a spiral roots or thick mass (root bound).
  • Are the size and shape of the seedling appropriate to the site?

Ask nurseries if they have added mycorrhizae treatments to seedlings. Mycorrhiza is a fungi that helps plants become less prone to water stress.

Ask how the plants have been hardened off. Hardening off seedlings exposes them to different weather. It prepares them for outdoor conditions.

If you’re not sure, ask nursery staff for help.

3. Prepare the site

The history of your site helps you know what site preparation is needed. When looking at the site, think about the following.

  • Is slash (leftover branches, vegetation) available to create rows to plant in?
  • Can woody debris be kept on site? This supports the growth of helpful root fungi known as mychorrizae.
  • Are nurse plants needed to help the young seedlings you're growing?
  • Some seedlings need lots of sunlight. Can you make canopy gaps to plant in?
  • Can you cultivate (break up) soils or debris to help water retention?
  • Does the site need weed control before planting?
  • Is mulch needed or is it available on site? Adding mulch to the surface of soil can stop moisture escaping.

4. Planting

Planting techniques are important for success.

  • Pre-soaking plants on site can help, especially if the soil is dry. Plants should never be planted when they're dry.
  • Watering after planting removes air gaps from the root ball. Roots can’t grow through air gaps, and they stop water travelling through soil.
  • To prevent water loss consider adding supplements to soil on tough sites.
  • Use plant guards to protect from animals, weeds or maintenance like herbicides.
  • Check if plant mulches are available to stop weeds and reduce water stress.

5. Maintenace

Poor maintenance is the reason why many native plants don’t survive.

Develop a maintenance plan for your site. Things to think about include:

  • managing weeds
  • mulching
  • protecting the plants from animals
  • whether ongoing irrigation is needed
  • pre-planting and ongoing pest control.

The 5 steps to planting success as supported by these guidelines.

Regeneration

Native regeneration can be more cost-effective than planting nursery-raised seedlings when planting tough sites.

Seeding or natural regeneration may be more feasible where:

  • the soils are more hostile. Planted seedlings tend to be less drought-resilient than plants grown from seeds on site.
  • sites are extremely harsh, as planted seedlings have low survival in these conditions.

Time and money can be saved if you allow regeneration after making sure stock can’t get on to the site and/or weed control.

Regeneration is most likely to work:

  • in areas with existing natural canopy
  • in some non-native canopies
  • next to existing leftovers of native forest
  • if plantations have been harvested in ways that keep native undergrowth.

In all these cases, natural regeneration can be a low-cost complement to planting.

One Billion Trees Programme research

This fact sheet was produced by Manaaki Whenua – Landcare Research. It is based on the technical report Improving resilience of native New Zealand woody seedlings to drought.

This research was commissioned by the One Billion Trees programme.

Read about other One Billion Trees science projects on the Ministry for Primary Industries’ website.