react this.refs is {}

mashForm.js

class MashForm extends React.Component{
  constructor(props){
    super(props)
    this.state = {malts: []}
  }

  componentDidMount() {
    this._fetchIngredients()
  }

  render(){
    return(
      <div>
        <form onSubmit={this.props.addHandler} className="form-inline" >
          <Select
            options={this.state.malts}
            ref='malt'
            onChange={this._onChange} />
          <Input
            type="text"
            placeholder="Amount"
            ref='amount'
            onChange={this._onChange} />
          <Input
            type="text"
            placeholder="Time"
            ref='time'
            onChange={this._onChange} />
          <Button type="submit">Add</Button>
        </form>
      </div>
    );
  }

  _onChange(){
    console.log(this.refs)
    let malt   = ReactDOM.findDOMNode(this.refs.malt).value;
    let amount = ReactDOM.findDOMNode(this.refs.amount).value;
    let time   = ReactDOM.findDOMNode(this.refs.time).value;

    console.log('onChange')
    this.props.handleChange({malt: malt, amount: amount, time: time})
  }

  _fetchIngredients() {
    $.ajax({
      url: this.props.url,
      dataType: 'json',
      cache: false,
      success: function(data) {
        this.setState({malts: data});
      }.bind(this),
      error: function(xhr, status, err) {
        console.error(this.props.url, status, err.toString());
      }.bind(this)
    });
  }
};

MashForm.defaultProps = {url: '/api/malts'};